A football team, a giant drinking straw, and a collapsed tank of water help David understand air pressure. This was graphically demonstrated in 1654 when Otto von Gueicke, Burgmeister of the town of Magdeburg, Germany used a vacuum pump to remove almost all of the air from the space between two half-meter diameter hemispheres.
Air pressure is created by the weight of the earth's atmosphere.
Although we can't see air, the gas molecules still have mass , and gravity acts upon it. The air pressure changes daily due to the heating and cooling of the earth's surface. When air gets warm, it expands, becoming less dense, and therefore pushes with less pressure. We can measure changes in atmospheric pressure by using a barometer . Some barometers use long glass tubes filled with mercury inverted in a dish.