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Materials:

*cardboard or wood (a small piece, maybe one inch wide and two inches long should be enough)
*scissors
*small pieces of soap
*a clean bowl filled with water (a sink filled with water will also work)

Method:

--1--Take your piece of wood or cardboard and shape it so that is resembles a boat. Something simple, like this shape:
______
| |______/ will work just fine.

--2--Cut a small notch in the back of the boat:
and place a piece of soap in the notch.

--3--Fill your bowl/sink (make sure it's clean!) with water. Place your boat into the water. It should propel itself forward.

Explanation:

Soap weakens the surface tension of water. Surface tension is a property of liquids that arises because of the strong cohesive forces near the surface of the liquid. The attraction between surface molecles causes the surface to contract and give it properties similar to those of an elastic membrane. In other words, surface tension is like a "skin" on the surface of water; it's what allows water bugs to walk on water. If you have ever blown bubbles before, you have seen how soap can weaken the surface tension of water. Mixing soap into water allows the "skin" to stretch, making it possible for you to blow bubbles.

When you put the soap in the notch in the back of your boat, it weakens the surface tension behind the boat. The boat is then pulled forward by the stronger surface tension in front of the boat.

Other things to try with your boat:

**Try making a notch that is not centered in the back of the boat. Make notchs to the right of the center and to the left. Take turns placing pieces of soap in each of these notches. What happens to the boat?
**Try and see if you can add rudders to your boat to make it turn.
**Have boat races. Fill a tub with water and make several boat. Experiment with different shapes and materials. Do skinny boats move faster than wide boats? Are short boats faster than long boats? Is a boat with a pointed front faster than a boat with a flat front?