This week's experiment comes from an email I got from a student. (Thanks Darius!) He wanted to know what causes your knuckles to pop. To try this, you will need:

- your hands

First, not everyone's knuckles will pop, and some pop more than others. Other joints in your body may also pop, some for the same reason as your knuckles, and some for other reasons.

There are different techniques for popping your knuckles, but they all work in basically the same way, by forcing the two bones of the joint to move farther apart. Probably the simplest way to do this is to interlace the fingers of your hands. Then turn your hands so that your palms are away from your body, and gently bend your fingers backwards. THIS SHOULD NOT HURT. DO NOT FORCE YOUR JOINT TO THE POINT WHERE IT IS PAINFUL. As the pressure builds, you should hear a pop from one or more of your knuckles.

If you search the internet for the answer to this question, you will find all sorts of answers, ranging from good explanations to wild guesses and pure fiction. The most comprehensive answer that I found was from "A bioengineering study of cavitation in the metacarpophalangeal joint" by
A. Unsworth, D. Dowson, AND V. Wright, from the Bioengineering Group for the study ofHuman Joints, the University of Leeds. If you want to read the article, you can find it here: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1005793/

If you don't want to wade through the article, the sound is caused by a process called cavitation. OK, so what is cavitation?

Lets think for a moment about the properties of liquids. Liquids take on the shape of their container, but they maintain their volume. You can't squeeze water into a smaller space, or stretch it to fill a larger space. If you try to stretch water into a larger space, at first, nothing seems to happen, but you are reducing the pressure of the liquid. When the pull gets strong enough, and the liquid pressure is low enough, some of the liquid changes to a gas, forming a bubble. Unlike the liquid, the gas in the bubble can be stretched into a larger space. That reduces the pull on the liquid, which raises its pressure. At that point, the gas in the bubble almost instantly changes back to a liquid, collapsing the bubble.

The same thing happens when you pop your knuckles. As you apply pressure to your knuckle joint, it forces the ends of the bones apart. Surrounding the joint is a liquid called synovial fluid. Moving the bones apart pulls on the fluid. If it was a gas, it would expand to fill the space as the bones separate. Since it is a fluid, it stays the same size, and its fluid pressure decreases. As you pull harder, the fluid's pressure gets lower and lower, until it reaches the point where the pressure is low enough to let some of the liquid change to a gas, forming a bubble. The bubble expands in response to the pull, which lowers the stress on the fluid. That lets the fluid's pressure go back to normal, which lets the gas change back into a liquid, and the bubble collapses. That collapse produces a loud sound.

Cavitation can be seen in other situations where liquids are subjected to very low pressures. Submarines have to be careful about cavitation from their propellers, as the sound can give away their position. An animal called the Pistol Shrimp snaps its claw fast enough to cause cavitation, producing a sound that is loud enough to stun or kill nearby fish.

Another question that frequently pops up with this subject is why you can usually only pop a joint once, and then you have to wait 20 to 30 minutes before it will pop again. When the low pressure causes the bubble of water vapor, it also causes some of the dissolved gases in the fluid to form bubbles. These bubbles, made up mostly of carbon dioxide, do not instantly collapse, remaining until the gases are reabsorbed back into the fluid. This usually takes 20 to 30 minutes. If you try cracking the joint again before those bubbles are reabsorbed, the bubbles of gas expand, preventing the pressure buildup that causes cavitation.

The other question that comes up frequently is whether knuckle cracking causes arthritis or other damage. Not many studies have been done, but those that have been done do not show any link between knuckle popping and arthritis.

Have a wonder-filled week.

Today we actually had some much needed rain. To celebrate the rain, I thought I would do an experiment that was related to rain.

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This week's experiment should be familiar to any of you that have ever cooked pancakes. As my mother taught me, and as you will find in most cookbooks, in order to tell if the skillet is hot enough for pancakes, you dip your fingers into some water and then shake a few drops onto the skillet. If the drops just sit there or if they hit the skillet and boil, then it is not hot enough. As the temperature of the skillet increases, you reach a point where the drop of water seems to bounce and glide around the skillet. Then you know that the skillet is hot enough for pancakes. This is called the Leidenfrost Effect, and that is what we want to observe now.

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This experiment was sent in by Leilah, an 11 year old list member from Indiana. It is exactly the kind of experiment I like, because it is simple, it makes you think, and it’s interesting enough to get you to actually try it, instead of just saying, "Wow, I'll have to try that some time."

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This project has science fair potential.

We are used to thinking of things as falling into the basic groups of solids, liquids, and gases. (In another experiment we will discuss a fourth state of matter, plasma.) In this experiment, we will examine a substance that sometimes acts like a solid and at other times acts like a liquid.

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There are many things that will float on water: pieces of wood, wax, Styrofoam, and many other things. They float because they are less dense than the water. Now for the question. Can water float on water? Is there a way to make water less dense?

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Why is half a water balloon different from a full one?

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This week's experiment came to me from my good friend Bob Cox. He told me about the trick and wanted to know why it worked. It took some thought and testing to come up with a theory of what is happening and then several e-mails to experts to confirm that I was on the right track. For this week's experiment, you will need:

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Can you lift an ice cube out of a glass of water with a string? Try it and see.

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Learn the science behind the Tricky Bottle!

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This week's experiment is a favorite of mine, which won't be a surprise to anyone that has been on my experiment list for long.  To try this tasty experiment, you will need:

• 2 glasses
• carbonated soda
• ice cream

For this experiment, we will make two ice cream sodas.  Although both will contain exactly the same ingredients, they will be very different.

Start by filling one glass half-full of soda.  Then add a scoop of ice cream.  In the other glass, start by adding a scoop of ice cream, and then pour the soda into the glass.

You should notice a big difference between the two mixtures.  The glass where you added the ice cream first will have lots of thick, long lasting foam, while the glass where the ice cream was added after the soda has very little foam.  Why is there such a big difference?

There are two things that contribute to the difference in the foam.  First, the carbonated soda contains quite a bit of carbon dioxide gas dissolved in it.  It may seem strange to think of a gas dissolving in a liquid, but it is quite common.  If you watched the Watched Pot video, you may remember the bubbles of gas that appeared before the water started to boil.  Those bubbles formed from gases that were dissolved in the water.

The soda is supersaturated with carbon dioxide, which means that it contains more of the gas than would normally dissolve in it.  As long as the soda is undisturbed, the carbon dioxide escapes very slowly, but you can speed up the process by adding bubbles.  The bubbles provide more surface area for the gas to escape from.  As the gas escapes, the bubbles grow larger, providing even more surface area.  

You can see a very good example of this with an unopened bottle of soda.  If you give it a hard shake, and then open it, you are in for a mess.  Shaking the soda introduces lots of tiny bubbles into the soda, providing plenty of places for the carbon dioxide to come out of solution.  On the other hand, if you shake the soda, and then let it sit for a while before opening it, the bubbles will have time to float to the surface and pop.  In that case, when you open the soda, it stays in the bottle.

Another example of bubbles causing foam is the classic Mentos and Coke experiment.  The candy has a porous surface, which produces many tiny bubbles.  You can get similar results by dropping pieces of chalk into your soda, as it also has a very porous surface, but I don't recommend drinking the soda afterwards.

Now back to our ice cream.  Ice cream contains a LOT of bubbles.  In fact, a carton of ice cream may be as much as half air.  Those bubbles are important, as they keep the ice cream soft and smooth, instead of hard and crunchy like an ice cube.  

When you pour the soda into the glass first, microscopic bubbles from irregularities in the sides of the glass serve as a starting place for the foam.  Most of the carbon dioxide bubbles form and pop before the ice cream is added.

OK, so why is it any different when you add the ice cream first?  The ice cream has lots of tiny bubbles of air, so much more of the carbon dioxide comes out of solution.  If you taste the soda afterwards, you will find that it is quite flat, with no fizz left.  But, ice cream also contains chemical thickeners, to make it smoother and creamier.  As you pour the soda over the ice cream, some of the ice cream melts, letting the thickeners mix with the soda.  Just as they thicken the ice cream they thicken the foam, making it much firmer, and much longer lasting.  Instead of quickly popping, this time the foam stays long enough for you to enjoy eating your tasty treat.
   
So although both recipes use exactly the same ingredients, the order you add them makes a big difference, although both turn out quite tasty.  And keep in mind that an important part of science is that experiments should be repeatable, so you might want to repeat the test several times, just to be sure of your results.

Have a wonder-filled week.

Make a diver that will rise and fall at your command, if you know the science.

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Ice Crystals

Grow your own ice crystals.

Learn some fun tricks with dry ice as we explore the science of sublimation.

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