This marvelous trick was sent to me by Josane. Besides the fun of trying it, I also had the fun of trying to figure it out, and designing an investigation to test my idea. To try this, you will need:

• a wall
• you

The trick is very simple. You stand facing a wall, about an arm's length away. Curl your index finger, and extend your arm straight out in front of you. Move until your knuckle is firmly touching the wall. Keeping your feet in exactly the same place, lower your arm to your side, and then raise it back up again to its original position. You will probably find that instead of touching the wall, your knuckle will now be about an inch short of touching the wall. Why? What happened?

Try it several times. Think about what is happening. Then try to come up with a way to test your ideas. You have until next week to figure it out, so this is a great opportunity to do some real science.

This activity is from my Experiment of the Week Newsletter. It is free, and will let you know about new resources on this site.

This time, we are going to take a look at a classic science experiment that has been used for a LONG time. In fact, it was already an old classic when I was a kid, and that was quite a while ago.

The experiment involves the link between our senses of taste and smell. Often it is done with apples and potatoes, but there is enough textural difference that you can often tell which is which. I have found that the results are much stronger with flavored candy. To try this, you will need:

• candy that has the same shape and texture, but comes in different flavors.

OK, lets begin with the standard experiment. Be sure that your pieces of candy will all feel the same in your mouth, and that they have distinctive flavors. If you don't see the candy, the taste should be your only clue to what flavor it is.

If you have a friend to help, then close your eyes, hold your nose, and have her give you one of the pieces of candy. Keeping eyes and nose tightly closed, put the candy in your mouth. You will taste a sweet taste, and probably some sour too, but you may be surprised that you can't tell if the candy is cherry, lime, orange, or some other flavor.

Then move your hand away from your nose, so you can breath normally. Yum! You get a sudden burst of flavor, telling you exactly what kind of candy it is. The reason for this is that your tongue has flavor receptors for basic flavors, such as sweet, salty, sour, bitter, and umami (the savory taste of meat.) Most of the other flavors that you taste are tied in with your sense of smell. If you can't smell them, then you don't taste them. That is why food tastes so bland when you have a cold.

But wait a minute! Your parents probably taught you to chew with your mouth closed. How can the smell get out of your mouth to go up your nose, so you can smell it? And your mouth stayed closed when you released your nose, but you still got that sudden burst of flavor. What is really happening?

Well, the smell of the food does have to reach your nose for you to taste all of those subtle flavors, but there is another path that those smells can take. Instead of inhaling those smells through your nose, you are exhaling them. As you breath out through your nose, your breath carries the smells from your mouth into your nose. You were not holding your nose to prevent you from inhaling the smells. Instead, you were blocking the way, so you could not exhale the smells through your nose.

Now that you are tasting the flavor, hold your nose again. After a second or two, the flavor disappears again.

So what if you just held your breath instead of holding your nose? Try that.

No, really. Try it and see for your self.

What did you find? Even holding your breath, you probably still tasted some of the flavor. Why? Think about what happens when you chew or swallow. Your mouth changes shape, your throat moves, your tongue moves around. All of that movement causes the air in your throat to move, forcing some of it up into your nose. It carries some of the smell to your nose even if you don't exhale. By blocking your nose, you pressurize it, preventing the air from your mouth from moving up.

OK, take it one more step. Hold your nose until the flavor goes away. Then release your nose, and inhale. While you are inhaling, keep your mouth closed. You probably won't taste the flavor. Then exhale. Ahh, there is the flavor again. The main path that the smells take to let you taste your food is up through the back of your throat. It is not inhaling that brings you the flavor. Exhaling is what gives you those wonderful flavors.

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.

The idea for this week's experiment came to me while I was moving our stuff into the hotel where we will be staying for the next 3 weeks. The room is on the second floor and we do not travel light. Basically we take everything except the kitchen sink. We would probably take that as well, but it is too hard to disconnect. After the 14th trip up the stairs, I started to think about muscles and how they get tired. One thing lead to another, and soon I had this week's experiment written. Why do your muscles get tired? And more important, why does your heart, which is also a muscle, not get tired? To find out, you will need:

- you

Your heart is a group of muscles which pump the blood through your body. To get an idea of its size, make a fist. That is about the same size as your heart. Pretending for a minute that your fist is your heart, lets take the idea a little farther. Open you hand about half way and then close it again. If you do that over and over, you can imagine that it is your heart beating. You can even make heart sounds (bump-bump) if you (bump-bump) want to. (bump-bump)

Keep this pretend heart beating as you read this. Soon your hand will begin to get tired. If you keep opening and closing your hand even after you are tired, it will begin to hurt. Why?

When you move your muscles, a chemical reaction takes place. Normally, this chemical reaction needs oxygen. We get this oxygen when we breath. The air moves into your lungs and the oxygen is absorbed by your blood. Your blood carries the oxygen to your muscles. As long as the muscle has plenty of oxygen, everything is fine and it can keep on moving.

If the muscle uses up oxygen faster than the blood can deliver it, then what happens? The muscle does not instantly shut down when the oxygen runs out. Instead, a different chemical reaction takes over. It lets your muscles move even if they do not have enough oxygen. The problem with this backup system is that the reaction makes a chemical called lactic acid. This acid irritates the muscle, making it hurt. If you overdo it, your muscles will be sore the next day. Keep overdoing it and you can damage the muscle.

If your heart is made of muscle, why doesn't it get tired? After all, your heart beats all day and all night, for your entire life. A large part of the answer has to do with blood. Your heart is between your lungs. Blood picks up oxygen from the lungs and flows directly to the heart. This insures that the heart always has plenty of oxygen, so it does not get tired. The one exception is if the blood vessels that lead to your heart get blocked. Then the heart muscles run low on oxygen and get tired. The pain that you feel is what tells you that you are having a heart attack.

Athletes exercise regularly to increase the blood flowing to their muscles. If the muscles get more blood, they get more oxygen. Then they can work harder and longer before they get tired. Right now, I could use some extra oxygen, so I could unload the rest of our stuff.

How do you feel? No, I don't mean are you happy or sad? Touch the back of your hand. Did you feel it? How? When you touched your hand, you pressed on nerves in your skin. These nerves reacted and sent a message to your brain, telling you that something touched your hand. Some parts of your skin have more nerves than others. This week, we are going to examine how these nerves are arranged. You will need:

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I thought this week we would do something on judging distances. If you look at the faces of different animals, you will find that their eyes are placed differently. Some have both eyes in the front of their face, like ours. Others have their eyes on the sides of their heads. Why? We can do an experiment to learn at least one reason for this.

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Through Rose Colored Glasses

What is it really like to look at the world through rose colored glasses? Not nearly as strange as when you finally take them off!

This week's experiment comes from a question that was sent to me by Hashi, one of the members of the Experiment of the Week list. She noticed that smells were stronger while taking a shower and asked why. To investigate, you will need:

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This week's experiment was one of those accidental discoveries that I love so much. All my life I have heard about the fact that the outer layer of your skin is made up of dead cells. These dead cells are your first line of defense against germs, harmful chemicals, etc. It is one thing to hear about this dead skin, but it is another thing to actually see it. I happened to be sitting in a beam of light when I scratched my arm and immediately knew what I would do for this weeks experiment.

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If you have ever played with a tuning fork, you know that they are interesting and fun, but they can also be expensive. For this week's experiment, we will use a regular fork from your kitchen to produce a beautiful tone. For this experiment, you will need:

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This project has science fair potential.

Have you ever noticed that warm food tastes different from cold food? If you have ever tasted cold pizza or melted ice cream, you know what I mean. This experiment will show you how temperature can effect taste.

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Have you ever seen a random dot stereogram? You see them on posters, cards, calendars, etc? It can be a random pattern of dots or a repeated pattern of just about anything, and if you stare at it in just the right way you suddenly see a 3 dimensional image sticking out of the page. Have you ever wondered how they are made? Lets take a look.

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