Gas molecules are constantly in motion, and when molecules move, they exert a force on their surroundings. Air pressure is all about that force– more specifically, the force per given area that gas molecules exert on their container. There are a lot of things that go into higher or lower air pressures. If you cram more gas molecules into a container, they move faster and exert more air pressure. Of course, air pressure is always present just because we live on a planet with an atmosphere. All those gas molecules are pressing down on you all the time, and you don’t even feel it! In this experiment, you’ll get to see atmospheric air pressure in action. You’ll need: 1 large glass jar with a few inches of water in the bottom 1 stack of cardstock paper 1 large bowl or container (in case things get messy) Instructions Hold a piece of cardstock over the mouth of the glass jar. Over the large bowl or container, turn the jar upside down. Remove your hand from the cardstock. Does the water stay in the jar? That’s because the air pressure inside the jar is less than the atmospheric air pressure pushing up on the piece of cardstock. Take away the cardstock. Does the water still stay in the jar? Of course not! The gas molecules from the air outside the jar rush into the jar and make the air pressure inside the same as the air pressure outside, so they cancel out and the only force with real power over the water is gravity. And we all know what gravity is capable of!