This is called Displacement of Liquids.

Next time you get into the bathtub, watch what happens to the level of the water.  The water makes room for your body and the water level rises!

This is called "displacement".  The amount of space something takes up is called its volume.

You can do an experiment in your classroom to see how water is displaced.   You will need a plastic bottle and a clear graduated measuring cup.

1. Fill the bottle to the top with water.

2. Stick your thumb into the bottle as far as it will go.  Some of the water will spill out.

3. Use the measuring cup to fill the bottle to the top again.  The amount of water you use is equal to the volume of your thumb.

Cool, eh???