I did this with my class a few weeks ago, to demonstrate that air takes up space.
1. Roll a small ball of tissue paper and stuff it about 1cm from the end of a straw.
2. Cover the other end of the straw with your finger.
3. Ask if the paper will get wet if you dip the straw right to the bottom of a tall glass of water.
4. After demonstrating that, ask what will happen if you remove your finger from the end of the straw.
5. Demonstrate that too. Ask for explanation before you explain.
Hope that helps