You will need:
- a bowl of water
Fill a large bowl with water. Cut a piece of thread about six inches long. Tie the ends together to form a loop and carefully lay the loop onto the surface of the water. Now challenge a friend to use only one finger and shape the loop into a circle. As they move their finger around, the loop will not cooperate, and they probably will not succeed. Instead, the string will stick to their finger. When they give up, make sure that the loop is still floating on the surface of the water. If it sinks, start with a fresh loop.
This is where the trick comes in. Before you start, rub your finger on a piece of soap or put a tiny bit of detergent on your finger. Be sure that the soap does not get onto the thread as you make the loop. When it is your turn to try to make a circle, simply stick the finger with the soap into the water on the inside of the loop. Instantly, the loop will move outward to form a circle.
Why does this happen? Water molecules stick to each other. They stick so well that at the surface they seem to form a "skin" that is known as surface tension. The thread floats on this "skin", but the water is also pulling on the thread. Since it is pulling equally in all directions, the string does not move.
When you put the soap into the water, the soap molecules hook onto the water molecules. This disrupts the surface tension. The water on the inside of the string is no longer pulling as hard on the string. The water on the outside of the string is still pulling just as hard, because the string acts as a barrier to keep the soap from spreading outside. The stronger pull of the water outside the string pulls the string outwards as far as it will go, forming it into a circle.
After you amaze your friend, be sure to explain the science involved. If you want to try the experiment again, you will need to start with a fresh, clean bowl of water, so that there will not be any soap at the start.