Air pressure is the force exerted on you by tiny air molecules. Even though you can't see air molecules, they still have weight and take up space. We are so used to the pressure that we don't notice it until it changes.

These are suction or pressure cups. They will illustrate that air is heavy and has weight. You can't see anything in between the 2 cups but air is present. When they are pushed together, you can hear the air being forced out. Now, try to pull them apart. It's difficult. The air in the room is holding them together. The weight of the air is called pressure. Air weights 14.7 pounds per square inch at sea level. By sliding the cups apart or cracking open the edge, air will be allowed to go back between the 2 cups making the air pressure equal on the outside and inside. This allows the cups to separate easily.

Pressure is balanced on the inside and outside of our body. Since we have air on the inside of our bodies, we stay firm and not squishy. When we change elevation, air pressure drops. One way our bodies maintain air pressure is through popping our ears. This allows the pressure between the inside and outside of our ears to remain equal. We also breathe faster. The number of air molecules decreases as air pressure decreases. In order to get the same amount of air molecules into our lungs, we need to breathe faster.

Have the students take a breath while putting their hands on their ribs. Tell them to observe what is happening to their chest. The students will feel and see their chests expand. Their lungs are filling up with air molecules just like a balloon fills up with air.