Articles Required : A cardboard piece, soap, a pan full of water.
• A boat driven by soap! Doesn't it sound strange? Well, not only it's possible, you can even make one yourself and simply surprise all your friends.
Cut out a boat-like figure from the cardboard piece, and fix two soap pieces at.its back as shown here. Now, if you want, your boat should follow a circular path instead of going straight, you can join a cardboard piece at the back to act like a rudder.
Well, your boat is ready. As you put it in a big pan of water, it will start sailing. How does it happen? As the soap dissolves in water, it reduces the surface tension at the back of the boat and due to the difference in surface tensions on two sides of the boat, it is pushed forward. But as soon as the soap gets dissolved in all of water the boat becomes stationary. Why?
Because of the dissolution of soap in the water/the surface tension of the whole body of water becomes the same. And thus the boat can't move forward any more. So it is suggested that once it happens, you change the water. And if you can go down to some pond etc. that would be ideal.