Take a ball of plasticene and drop it into a beaker of water – it will sink. The density of plasticene is greater than the density of water. Now take the SAME ball of plasticene and make a small hollow boat from it. This can now be made to float on the water. The boat floats because the average density of the plasticene and the air within the boat is less than the density of the water.
Apparatus required: Plasticene Plastic tank or bucket of water