Fill a tub or small aquarium with water (the water should be deeper than the cup you're planning to use for this activity).
Crumple up a sheet of paper and place it in the bottom of a cup (use enough paper so it will stay in the bottom when the cup is turned over.
Hold the cup upside down and push it into the water.
Pull the cup up and out of the water. Remove the paper. Is it wet or dry? Why?
The cup was filled with air. When you pushed the cup into the water, you trapped the air in the cup. The air pushed on the water, pushing it out of the way of the cup. As a result, the paper stayed dry.