This week's experiment came as a surprise to me. I got an e-mail, asking for an explanation for one of my favorite science tricks. Because I get so many e-mails with science questions, I often answer them by sending a copy of a past experiment that covered that subject. After so many experiments, I sometimes forget what I have written about. As I looked, I was amazed that I could not find a past experiment that used this simple science trick. Now I have to go back and see what other favorites I have missed.

Materials:

Plan:

Science is just the opposite. A science entertainer gets to amaze you with a dramatic demonstration, and then gets to amaze you again by showing you the science that makes it happen. That is the important part, and to me the most fun.

Before you start this demonstration, rub the end of your finger gently across a bar of soap. You want a little soap on your finger, but not enough to be visible.

Place a bowl of water on the table. Sprinkle some pepper across the surface of the water. At this point, a magician would tell a story about an ancient warrior that was so fierce that even inanimate objects were afraid of him. It is said that even flakes of pepper would run from him.

Then you ask your audience to try scaring the pepper. Have them make a scary face and stick their finger into the bowl of water. Unless one of them knows the trick, or was sloppy in washing his hands, nothing will happen. When it is your turn, you put on a fierce face and stick your finger into the water. Be sure to use the finger that you rubbed against the soap. Instantly, all of the pepper will rush away from your finger, towards the sides of the bowl.

At this point, the magician would go on to the next trick, but the scientist is just getting started. Why does it work?

We start with an understanding about water. Water molecules are very sticky. They have a strong attraction to other water molecules. In the center of a glass of water, the molecules are sticking to other water molecules in all directions.

At the surface, there are no water molecules above for them to stick to, so they stick even more to the molecules beside them. All of the water molecules are pulling towards each other, much like a lot of magnets attracting each other. This forms a "skin" at the surface of the water that we call surface tension. This surface tension is strong enough to support insects like the water strider. They actually walk on the surface of the water. It will even support a steel needle if you place it on a small piece of tissue paper and gently lower it onto the water. The tissue paper absorbs water and sinks, but the needle floats, supported by the surface tension.

There are ways to break this surface tension. One way is by using soap. A soap molecule will stick to a water molecule, keeping it from sticking to other water molecules. That is what happens when you stick your soapy finger into the water. The water molecules beside your finger suddenly stick to the soap molecules instead of each other. The other water molecules on the surface are still pulling, so it is much like a game of Tug-of-War where one side suddenly lets go of the rope. The water at the edges is still pulling, but the water in the center is not pulling back. The surface molecules and the pepper floating at the surface are all pulled quickly to the sides.

It is important to remember that even a tiny bit of soap will cause this to happen, so if you want to let your audience try it themselves, you can't reuse the same bowl, unless you rinse it very well and start with fresh water.