For an easy-to-make model of air pressure, fill a glass with water. Place a piece of cardboard over the top of the glass and hold the cardboard in place with one hand as you turn the glass upside down. Make sure the cardboard does not bend. Now remove your hand from the cardboard. What happens? Does the cardboard fall off and the water flow out? No, the cardboard remains on the glass. Why? The air pressure pushing up on the cardboard is greater than the weight of the water pushing down. For fun, turn the cup sideways, and then turn it all around. The cardboard continues to stick!