a container of water for the boat to move across (a sink or tub will do)
How to do it
Cut a boat out of the index card.
Cut a small slot in the rear of the boat for the engine. That's where the fuel (the dishwashing soap) will go.
Float the boat in the container of water.
Pour a few drops of dishwashing soap in the engine and watch it go.
Why did the boat move? The attraction between water particles at the water's surface produces surface tension. This gives the surface a skin-like quality. Think of this thin skin-like quality like a balloon. Adding liquid soap is like popping the balloon. You break the surface tension; the thin "skin" breaks and pulls away to the sides. If there's something floating on these water particles, like the boat, it goes along for the ride.
So, if the soap makes the surface of the water change, you should be able to get the boat to move just by putting the soap on the water rather than on the engine. Try it out.
Use fresh water each time you try this or it won't work.
For another fun activity that demonstrates surface tension, check out ZoomSci's Floating Paper Clips.