An Air Pressure Demonstration
Step 1: Materials
Find a piece of construction paper and a flat lidded cup with water in it.
Place paper on top of lid of the cup.
Apply pressure to the paper on top of the cup. Flip upside down and release your hold on the paper.
Step 4: Explanation
Air pressure is the force exerted by air. In this experiment since the air pressure is greater outside of the cup than inside the cup the outer air pressure keeps the cardboard in place. The higher pressure outside acts on the lower pressure inside the cup.