1. Cut the index card into the shape of a boat with a notch in the back, just like the diagram.
2. Fill a shallow pan, sink, or bathtub with water and set your boat on the surface.
3. Pour a few drops of dish soap in the notch at the back of the boat.
Surface tension is the way water molecules are more attracted to each other than to the air. When your boat sits in the water the surface tension is the same on all sides. When you put the drop of soap near the back, however, the soap molecules lowers the water’s surface tension. The force of the surface tension pulling on the front of the boat is now greater than the force pulling behind, so the boat moves forward. (Make sure you use clean water if you try again; it won’t work if the water is already soapy.)
Learn more about surface tension with these fun surface tension experiments.