How do you light the inside of a windowless room? With a bottle of water, of course! Driven by the outrageous price of electricity in the Philippines, Illac Diaz developed this economical and environmentally-friendly alternative. And in just four months, more than 15,000 bottles have been installed. How does it work? Imagine cutting a hole in the roof. Light enters, but so does the rain. Now plug the hole with a bottle of water. Sure, the bottle allows light in and not the rain, but the sunlight entering the bottle refracts in the water, spreading out into the room below! See these water bottles in action here.

Originating in the elaborate courts of the Italian Renaissance in the 1400s, it developed into an art form during the 17^th century in France under the reign of Louis XIV. But the elegant arabesques, exhilarating grand jetés, and energetic fouettés en tournant of ballet that we know today grew out of the Romanticism of the late 1800s, when ballerinas portrayed fairy-like creatures and almost seemed capable of floating in the air.

Have you ever wondered how a dancer can seem to defy gravity while your own feet are so firmly planted on the ground? Let’s take a closer look at gravity and discover the secrets behind some of these beautiful movements.

An article about dance should make you move, right? Here’s an experiment. Stand against a wall with your heels touching the wall. Now bend over and touch your toes. Did you make it?

Why did it suddenly become so difficult to do this simple task? The problem is the location of your center of mass: the average location of all the matter in your body. When standing straight, your center of mass lies within your body just below your belly button. It is situated above your feet, so you have no trouble standing.

But your center of mass can move depending on the position of your limbs. What if your body is horizontal? In a pas de deux, a male dancer may lift a ballerina into the air. Have you ever noticed where his hands are placed? Rather than clasping her waist or her thighs, he clasps her hips. Not only do her hipbones provide support, this is the point where her body is balanced; this is the location of her center of mass. Any closer to her belly or her thighs and the balance is upset; her center of mass is no longer above him, and he won’t be able to hold her aloft.

If your body is not straight, however, your center of mass is different. Try the experiment again. What happens to your center of mass as you bend over? By moving your torso away from the wall, you move some of your mass forward. Your center of mass also moves forward to a point somewhere in front of your upper thighs, not inside your body! Continue bending forward and at some point it will have moved out beyond your toes and you risk falling over. What do you do to keep from landing face-first on the floor? Try it and see. Can you figure out why standing against the wall makes such a difference?

Dancers instinctively know that to execute a perfect turn, their center of mass needs to be over their feet. Imagine trying to execute a turn, like a pirouette, on one foot. What happens if your center of mass is not directly over your feet? You end up with a wobbly turn, or worse, falling over!

The center of mass also helps explain those big leaps, or grands jetés, that seem to hang effortlessly in the air. After taking a few quick steps, a dancer takes a leap, one leg leading and one trailing. During the leap her head rises a certain amount. (For some numbers and more physics, check out Dr. George Gollin’s Physics of Dance website.) As soon as she starts the leap, however, she raises her legs, i.e. some mass, to be parallel to the floor, which raises her center of mass some more. During the jump, her center of mass rises more than her head, making the leap seem even higher. What is more, her center of mass moves the most at the very beginning and end of the jump; it moves very little through most of the leap, giving the impression she is hanging in the air or floating.

We are so familiar with gravity’s pull attaching us to the ground that the light airy movements in ballet take us by surprise. The movements seem much more possible, however, when we apply a little physics and see how ballet has developed to work with gravity. The result? An art form that becomes even more exquisite!

PS. Here is another experiment. Sit in a chair, with your knees at a 90° angle, your feet flat on the floor. Now stand up without moving your feet or bending forward. How did you do? Our natural instinct is to bend forward, but try it again, without bending forward! When you sit in a chair, your center of mass lies at a point above your thighs in front of your lower belly. It is impossible to stand up in the second experiment unless you move your center of mass closer to your feet, which is what happens when you bend forward.

As part of the Apollo program, United States astronauts (one was also a geologist) walked on the Moon six times between 1969 and 1972, the only times humans have set foot on another celestial body. Will we ever return? As the BBC describes, NASA has released high-resolution images of the Apollo landing sites, all located on the near side of the Moon. The Lunar Reconnaissance Orbiter Camera has been orbiting the Moon since 2009 at an average altitude of 31 mi (50 km) to gain information about potential future landing sites, and was able to move into an orbit bringing it just 13 mi (21 km) above the surface. Think the astronauts left the Moon in pristine condition? The pictures reveal tire tracks, astronaut boot prints, and discarded pieces of equipment.

Share your thoughts: Would you go to the Moon if you had the chance?

Did you catch any? A couple of weeks ago, many eyes were turned on the sky to witness the annual Perseid meteor shower where, under the best conditions, viewers can see a meteor as often as every minute. Though meteor showers have been observed since antiquity, it wasn’t until the mid-1800s that they were recognized as astronomical events. But what are meteors and why the name “Perseid”? To understand, we need to step back 4.5 billion years, to the beginning of the Solar System…

It all began with a massive cloud of gas and dust collapsing under the force of gravity, flattening and spinning faster and faster as it got smaller. A star, the Sun, formed at the center and the gas and dust revolving in a disk around the star coalesced into objects of various sizes: the familiar eight (sorry!) planets, their moons, the minor planets and asteroids, and other smaller chunks of rock and/or ice.

While the largest objects are nearly spherical and move smoothly in their relatively circular orbits, comets are the unruly hooligans of the Solar System. These irregular chunks of rock and ice formed in the farthest reaches of the Solar System. Close encounters with more massive objects flung them into highly eccentric — very non-circular — orbits, carrying them extremely close to the Sun. Notice how oblong the orbit of Halley’s comet is, as well as how close it approaches the Sun. As a comet nears the Sun, heat from the Sun causes frozen gases to vaporize, releasing dust and debris in the process. The gases, which we see reflecting light from the Sun, surround the comet and stretch out in a tail due to the Sun’s radiation pressure and solar wind; the dust and debris remain behind in the comet’s orbital path.

This rocky debris left by the comet is just one source of meteoroids — called meteors when they hit the Earth’s atmosphere — but it offers the most eye-catching display for us here on Earth. If the orbit of a comet crosses Earth’s orbit, the Earth will travel through the stream of rocky debris left by the comet. Thus, the Perseid meteor shower occurs when the Earth passes through the debris trail of the comet Swift-Tuttle just as the Orionid meteor shower, occurring in late October, is due to debris from comet Halley.

And the name of a meteor shower? We have to be good observers and notice that all the meteors in a shower come from the same point in the sky, the radiant point, which lies in a region, or constellation, of the sky. So, the Perseid meteors seem to come from the constellation Perseus, and the Orionids from the constellation Orion.

When meteoroids hit the Earth’s upper atmosphere, molecules in the atmosphere and those stripped off the surface of the meteor become ionized — they lose electrons. When atoms recapture the electrons, light is emitted. Aside from a pretty show, the color of this ionization trail gives us clues to the meteor’s composition; most meteors are yellow or green, meaning they contain a lot of iron or copper.

But scientists often want to know specifics: how much iron or copper is in each meteor? Practically all meteors disintegrate before they reach the lower atmosphere, so it is a challenge to have a meteor in hand. However, some meteors do hit the ground — they are called meteorites — and, if recovered, offer us a glimpse of the very origins of the Solar System!

As the meteorite speeds through the atmosphere, its outer layers melt, but the interior remains as it was when the meteoroid formed. Most recovered meteorites contain a lot of iron and nickel, which were used by early civilizations to make tools. Far more common are stony meteorites, which look like… well… stones. Because of this, they are difficult to identify. But, if you find one, you will most likely be holding a chunk of a primitive asteroid! The asteroid probably collided with another hunk of interplanetary rock, forming a meteoroid that went hurtling through the Solar System, through the Earth’s atmosphere, and, after some years, wound up in your hand!

What’s more, that meteorite can give us some very useful information. For example, when it formed, it contained two isotopes of strontium: ⁸⁶Sr and ⁸⁷Sr, which has one extra neutron. Over a characteristic length of time, that extra neutron turned into a proton, creating a different element, rubidium (⁸⁷Rb). If we compare the ratio ⁸⁷Sr/⁸⁶Sr to ⁸⁷Rb/⁸⁶Sr, we get 4.6×10⁹ years. Sound familiar? Not only is this the age of the meteorite, but since meteorites are among the oldest objects in the Solar System, it also gives the age of the Solar System!

If you missed the Perseids or you simply weren’t aware of it, keep looking up. Your next chance is just a couple of months away. The Orionids occur in late October, and the Leonids show up in mid-November. Here’s to clear skies!

Yesterday a quick rainstorm came through.  Shortly after it passed, a beautiful rainbow appeared in the sky, a perfect arc that touched down somewhere this side of the distant hills. While my 2-year-old nephew thought the rainbow very pretty, he would have none of the explanation, so I offer it here to you.

In our last post, we talked about why the sky is blue, and in particular, we mentioned that light from the Sun contains “all the colors of the rainbow.” We also mentioned that the atmosphere contains water vapor, the amount of which is obviously higher just after a rainstorm.

To these two elements we add a third: the refraction of light. Light refracts, or changes direction, when it enters a different material, and the amount and direction of refraction depends on a property of the material called the index of refraction. For example, light traveling from air into water refracts less than light traveling from air into glass because the index of refraction of water is lower.  Refraction depends on the wavelength as well, with blue light refracting more than red light. This is known as dispersion, and it also depends on the material.

So given some sunlight, a raindrop, and a bit of physics, what happens? The figure below shows a raindrop. See how the colors separate when sunlight enters the raindrop? This results from refraction and dispersion. Look now at what happens when the light in the raindrop tries to get out the other side. It reflects! If light incident on this part of the raindrop is not perpendicular enough to the surface, it will reflect instead of refract; this is known as internal reflection.

So light enters the raindrop and reflects off the inside, hitting another part of the raindrop. This reflected light now refracts a second time when it leaves the raindrop (the light is more perpendicular to the surface this time). Blue light again refracts more than red light, meaning the colors spread out even more.

If you’ve looked carefully at the diagram, you’ll notice the blue light ends up on top. But rainbows don’t look like this! To observe a rainbow, you need to stand with the Sun at your back, facing the raindrops. Looking straight ahead, there is a band of raindrops where the blue light is directed at your eyes. The raindrops further up will be in a position where the green light reaches your eyes, and finally the raindrops even higher are in a position where red light reaches your eyes. So we see red on top!

What about that pot of gold? I’m sorry to say you’ll never reach it. The rainbow’s location is not fixed, but rather depends on the your position and that of the Sun. If either you or the Sun moves, so does the rainbow! In fact, the rainbow’s semicircular shape also depends on these positions. Sunlight reflects off of all the water droplets in the sky; it just happens that the reflected light that reaches your eyes comes from a position 40-42° from the line connecting the Sun and your head. (This diagram may help clarify things.) Consequently, rainbows can only be seen when the Sun is within 42° from the horizon, or if you are high off the ground.

And double rainbows? This is caused by light internally reflecting twice within the water droplet. This secondary reflection also causes the colors to be inverted, with red on the bottom and blue on the top. Between the two rainbows lies a dark band known as Alexander’s Band, after Alexander of Aphrodisias, the Ancient Greek philosopher who first described it.

And last question… Why mention diamonds in the same article? Because they sparkle for pretty much the same reasons!

“Not that I give a hoot about jewelry. Diamonds, yes. But it’s tacky to wear diamonds before you’re forty.” ~Holly Golightly, Breakfast at Tiffany’s

Diamonds have high dispersion, which separates the colors a lot giving diamonds that characteristic fire. But diamonds can’t sparkle if light doesn’t internally reflect twice and exit out the top of the diamond. This depends on the index of refraction: the higher the index of refraction, the more likely it is that light will be internally reflected in a material. Diamonds have a very high index of refraction, making internal reflection very easy. This is where the cut of the diamond becomes important. If the bottom angle of the diamond is too shallow or too deep, light will not reflect, or it will only reflect once, getting lost out the lower side of the stone, as in the figure below. The magic number for this angle, as it turns out, is about 98°, depending on the way other parts of the diamond are cut.

The next time you’re shopping for diamonds (maybe before you’re forty), you’ll now know why your purchase should have the right cut. The things nature designs are often inspiring, but it’s always amazing how they can become truly dazzling with just a little bit of tweaking by human hands!

It’s a very common question, and you’ve probably asked it at least once yourself, but do you have the answer?

To explain, let’s start with the source of the light: the Sun. The Sun emits a lot of energy – light – spanning the electromagnetic spectrum from X-rays to radio waves. Most of this is either visible light (44%) or infrared (48%) radiation. Much of the remaining 7% consists of ultraviolet light. Wait… only 7%? I know what you’re thinking. Even though it makes up only a small fraction of the Sun’s light, we most often hear about this type of light because it is so damaging to our skin.

The Sun emits all colors – wavelengths ­– of visible light, from short blue wavelengths (0.4 mm) to long red wavelengths (0.7 mm), but it emits more of some colors and less of others. If we plot the intensity, or amount of light, versus the wavelength, we end up with the figure shown below, called a spectrum. The familiar colors of visible light are shown as well for comparison with the wavelength. What do you notice? The Sun emits all the colors of visible light, but the color it emits most is green. But the Sun doesn’t look green! All those colors emitted by the Sun get blended together and the result is a Sun that appears white from outer space.

This light leaves the Sun and speeds along its 8.3-minute trip to the Earth, where it hits the atmosphere. The atmosphere is composed of different atoms and molecules, most of which are nitrogen (N­­₂, 78.1%), oxygen (O₂, 20.9%), and argon (Ar, 0.9%). The remaining 0.1% is made up of a mix of different trace gases like neon (Ne), helium (He), methane (CH₄), carbon dioxide (CO₂), and ozone (O₃). There is also some water vapor in the air, about 1–4% at the Earth’s surface, as well as some dust.

When sunlight hits the atmosphere, it interacts with the particles in the air, and the way it interacts depends on the size of the particles. For large particles like water vapor or dust, all wavelengths of light reflect off the particles equally. The interaction of light with smaller particles, however, is much more dramatic. This interaction is called Rayleigh scattering. When light hits a particle, the particle absorbs the light and then releases it in a different direction. Rayleigh scattering depends strongly on the wavelength of light, which means that shorter wavelength, blue light is scattered much more than longer wavelength, red light. Blue light gets scattered in all directions, so it reaches your eyes from whichever part of the sky you view. Red, orange, and yellow light gets scattered less, so if you glance at the sky near the Sun (don’t look directly at the Sun!), that portion of the sky will look yellower.

Okay, so if the Sun emits more green light, why isn’t the sky green? It isn’t green for the same reason the Sun itself isn’t green: the colors that scatter the most create the blue color you see when they’re blended, or averaged, together.

And if shorter wavelengths are scattered more, why isn’t the sky purple? This is simply because there isn’t much purple light coming from the Sun. There is much more blue and green light making the average scattered light appear blue. But here’s an interesting thought. If the surface temperature of the Sun were about 1500 K hotter, we would have a purple sky!

Come back next week for a physical description of how rainbows form and why diamonds sparkle!

On 8 November 1895, Wilhelm Röntgen discovered an unknown type of electromagnetic radiation. He called this radiation X-rays, using the mathematical symbol x to represent something unknown. Not only did he win the first Nobel Prize for his work in 1901, he also ushered in a new era in medicine, one where it was no longer necessary to cut open the body to investigate an ailment. In fact, he unwittingly realized this potential early on when he used his wife’s hand to make the first X-ray image. With the devastation of World War I and II, X-rays became widely used and have become a vital instrument for doctors and dentists ever since.

Compared to today, medical diagnosis in the late 1800s was very primitive. To investigate ailments, doctors were limited to their own five senses. The senses of sight and smell were able to detect exterior signs of disease. (We hope they didn’t taste their patients too often.) Investigating the interior without dissection the body was more difficult. The sense of touch helped with broken bones or foreign objects lodged within the body, but swelling at the site could make diagnosis difficult. Aiding hearing, the stethoscope magnified sounds in the body. However, diagnoses relying on touch or sound were always dependent on a mental image the doctor created of the patient’s innards. And this mental map could look quite different from reality.

With X-rays, however, doctors could make a real image of a patient’s insides, which, as you can imagine, greatly improved medical treatment.

In our post about archaeology, we mentioned the electromagnetic spectrum and some uses of infrared radiation — radiation with wavelengths slightly longer than visible light. In this post, we jump to the other end of the electromagnetic spectrum. X-rays have wavelengths much shorter than visible light.

In order to make an X-ray image of a patient’s insides, doctors first need a source of X-rays. To make X-rays, a heated piece of metal called the cathode, and a collector — a metal plate called the anode — are placed within a glass tube from which all the air has been removed. The cathode and anode are connected to a high voltage power source and a beam of electrons is created in the tube between the cathode and anode. X-rays are produced when electrons hit the anode. This setup, called an X-ray tube, is nearly identical to the cathode-ray tubes used in older television sets and computer monitors. (In fact, the word set refers to the set of cathode-ray tubes making up the television.)

In medical uses, a tube like this is used to create short pulses of X-rays that are aimed at the patient. Some photographic film is placed behind the patient. X-rays are blocked somewhat by denser materials like bones and pass more easily through less dense materials like tissues. A shadow forms on the film where X-rays are blocked. When developed, the film turns darker where more X-rays have hit it, so denser objects like bones appear lighter on the final X-ray.

As useful as this simple technique is, more advanced techniques have also been developed. A CT (computed tomography) scan, for example, is a series of images made by passing the X-ray tube in a circle around a patient. Thousands of images are made from many directions and these images are then compiled to form a 3D image.

Now, the next time you visit the dentist, or go through security at an airport, or break a bone, you can thank Mr Röntgen for his discovery!

Tanis was once the thriving capital of Ancient Egypt.  Between the 11^th and 8^th centuries BC it was a large, rich city, its economy based on commerce, its proximity to the sea making it an important post for seaborne trade throughout the Mediterranean.  The prosperity of the city accounts for the construction of the largest temple ever built in Egypt, dedicated to the chief deity Amun.  Three thousand years later, however, Tanis is nothing more than an undeveloped stretch of land to the east of modern San-el-Hagar, scattered here and there with ruins and partially excavated areas.

At least, this is how we see it with our own eyes.

I am humbled by having had the opportunity to attend the World Economic Forum in Davos this past week. During my time there, I learned that most of the interesting activity at Davos happens in the coffee lounges and hallways of the Congress Center where the interconnectivity of people causes serendipitous encounters which lead to future collaboration. The dinners and parties are great places for networking and for probing some of the most brilliant minds on topics affecting our times. Most of my friends would not agree with me, but one cannot ignore the amount of quality information being debated at the more formal lecture-type gatherings. I think most people told me they avoided a lot of the lectures and preferred talking to colleagues and new people outside the lecture halls.

The conference is perfect for people who like to learn a little out of everything since the topics of the lectures are as varied as politics of India, Climate Change, Music, Psychology, physics, and many on the current state of the world’s economy. But if what you want is a deep analysis of each of the topics, this is definitely not the place. During many of the panels, each presenter gets at most 10mins to explain their point of view and by the time everyone has exposed their opinions, the session is almost over. I quickly found that so many things compete for your attention and I for one at times felt guilty that I was missing some other interesting talk while I sat listening to a different one. A scattered, ADD-like state very akin to what we experience culturally nowadays with a multitude of platforms, friends, jobs, family, trips competing for our attention.

Nevertheless, here are some lectures that I found interesting:

Insights on China addressed the Export Policy, Real Estate, Central government policy-making and private equity of the region. The Crystal Award Cermony presented an award to Jose Carreras for his work on Leukemia awareness. He said “It is our duty to use our popularity in order to give as much back to society as we receive from it.” Celebrated music director A R Rahman received the Crystal award and Carreras sang the Passion concert for us. What a treat. The Opening address was given by Dmitry Medvedev, President of the Russian Federation, who read the peech from his iPad.

I attended a dinner talk on “Social Media Addiction” with Dan Ariely, Charlie Beckett, Trevor Doughert Reid Hoffman, Diarmuid Martin, Marissa Mayer and Clay Shirky, moderated by Loic LeMeur. Fantatstic table discussions and insights into this phenomenon by simply trying to answer the question of why is addiction to social media bad? Loic did a wonderful job at moderating the conversations.

Larry Krauss (Physics Professor at Arizona State) and Rolf-Dieter Heuer (Director of CERN) lectured on the Universe, a visual exploration of Astrophysics and Quantum physics. Moderated by Philip Campbell (Editor in Chief of Nature magazine). I liked George Soros and Christine Lagarde’s panel on an International Monetary System where they discussed lessons from previous financial crises, enhanced G20 coordination and modalities of international monetary system reform. I missed the talk by pilot/her Chesley B. Sullenberger III who spoke on Leadership under Pressure. I later heard his talk was amazing.

I would have also loved to attend the talk on Science, Discovery and Controversy with Francis Collins, Director of the NIH, Howard Alper Chair and President of Science, Technolloogy and Innovation in Canada, Dan Esty (Yale Professor) Larry Kraus and J. Craig Venter, Founder and Presidnet of the Venter Institute. But it occurred simultaneously with the social media addiction one which ended up being great.

I went to hear Bill Clinton speak, as usual charming the crowds with insights into American politics, society and the economy. I attended a lecture on Global Climate Change from a political perspective with Presidents Jacob Zuma (South Africa), Felipe Calderon (Mexico), Denmark’s Connie Hedegaard, the EU commissioner for climate action and Costa Rica’s Christiana Figueres who in my opinion was the sharpest of them all. Hedegaard said that China, once an unrepentant polluter opposed to global controls, had joined Europe in deciding to invest in clean fuel technology in order to steal a march on America in a lucrative future market.

I went to a lecture on Smart Mobility that discussed how the integration of information, telecommunication and transportation technology will change our future mobility. I escaped 15 minutes before it ended to attend Ian Bremmer’s (President of the Eurasia Group) panel on Managing “Black Swans” and “Fat Tails”. Ian did a great job but I thought the panel was too diverse focusing on too many kinds of risk such as financial, energy, environmental, political, sovereign default, etc… so the conversation did not arrive at any significant points.

My friend Itay Talgam gave one of the most fantastic talks by focusing on an analogy between orchestra conductors’ style and CEO leadership. He showed videos with examples of different styles of conducting an orchestra and its effects on the music and compared them to different business leadership situations. He was warm, funny and very engaging. Everyone in the audience laughed and even asked for more once the allotted time had finished!

Finally, one of my favorite lectures was on “The Science of Emotions” which tackled the question on “How can we master emotions for a happier, less stressful and more productive life?” Tania Singer, a neuroscientist did a superb job at explaining how there are (approximately) three parts of the brain: incentive focused, non-wanting affiliation focused and threat-focused. She talked about the correlation between different parts of the brain such as the amygdale and different emotions. She discussed experiments with sniffing natural oxytocin and how it causes people to trust and care for one another more. I can’t wait for that drug to be out in the market! The only thing is that the effect only lasts for 20 minutes. Daniel Goleman (Emotional Intelligence) talked about life-work balance and managing emotions in big corporations. There was also a female Buddhist Priest, Roshi Joan Halifax, who discussed the experiments measuring compassion and happiness in Buddhist monks.

We eat them cooked any one of half a dozen ways in the morning.  We use them in cakes and cookies and as a meringue on lemon pies.  The particularly ambitious cook may use them in a mousse or a soufflé.  (For eggs in cooking, revisit Physics in the Kitchen.)  But have you ever stopped to think about how amazing the egg really is?

We all know that eggs should be handled carefully because their shells are incredibly thin.  Drop an egg a short distance and you have a gooey mess to clean up.  One simple tap on the edge of the counter is enough to crack open the shell.  But try this experiment: hold an egg in the palm of your hand and curl your fingers around it.  Now squeeze with all your might.

Did it break?  If you didn’t believe me and didn’t squeeze with all your strength, go back and try again.

What on Earth…?  The key to an eggshell’s strength is the fact that its whole surface is curved.  The strongest shape is a sphere, and an egg is a close approximation of this.  (The reason it’s not exactly a sphere in just a moment.)  With no corners or flat sides to weaken it, the forces you apply to the egg by curling your fingers around it are distributed equally over the egg rather than concentrating at any one point. 

This works even in the following way. Hold the top and bottom of the egg with your thumb and forefinger and squeeze.  Did it break this time?  In physics terms, we say that the egg has a high “compression strength” — you’re compressing the egg, but it doesn’t break!  In fact, an egg has such high compression strength, that (with the proper setup) it can support the weight of a small personwithout breaking.

So why aren’t eggs spherical?  The oval shape is created as the bird lays it, and this turns out to be an advantage for the hen.  The shape prevents the eggs from rolling away!  Spherical eggs would roll and roll and roll… and never return.  For birds like ostriches that nest on the ground, this isn’t an issue, and their eggs are generally more spherical.  But birds that nest on cliffs often lay very conical eggs, which roll in a tight circle around the narrow end and remain on the ledge.

What else?  How can you tell if your egg is fresh?  Eggs contain an air pocket that forms when its contents shrink as it cools after being laid.  As the egg ages, moisture evaporates and the air cell grows larger, reducing the average density of the egg.  An object floats in a liquid if it is less dense than the liquid and an object denser than the liquid sinks.  We can therefore use the egg’s density as a handy measure of the egg’s freshness!  Here’s how it works.  Place your egg in a container of water.  A fresh egg with a small air pocket will rest horizontally on the bottom.  The air pocket in a 1-week-old egg is slightly larger — its density is less — and the end will hover slightly off the bottom, and an egg that’s 2-3 weeks old — even less dense — will rest vertically on the bottom.  Don’t eat any eggs that float!

Now that you know so much about eggs, here’s a bonus question: what do eggs and Roman arches have in common? 

PS. When squeezing your egg: don’t wear any rings, and make sure your egg doesn’t have any cracks already.  My “research” egg was blessed with a crack and I ended up with a handful of broken shell and raw egg oozing between my fingers and onto the floor!