Why do some things float and others sink?

You need:

• modelling clay
• a beaker
• water

What to do:

1. Fill the beaker with water.
2. Mould the modelling clay into a boat and place it in the water.
3. Now roll the boat-shaped clay into a ball. Place it in the water.  

What happens?

The little boat settles on the top. The ball sinks to the bottom.

Reason:

If more water is displaced by the object, greater will be the thrust which the object gets towards the top. With the clay boat, a wide surface floats on the water and therefore it displaces more amount of water. Hence it gets an upward thrust which is sufficient to keep it afloat. Where as the clay ball displaces very little water. So the thrust which it gets is not enough to keep it afloat. This experiment shows that floating also depends on the shape of an object.