 Gases are a physical state that you walk through all the time. You probably don't even notice that you're being bombarded by trillions and trillions of nitrogen and oxygen molecules each second. Or that you have a column of air that extends the top of the atmosphere pushing down on you at all times. In this chapter you will learn why one of these balloons floats and the other one sinks. You will learn why gases expand when you heat them. Why Tony sinks when the bottle is squeezed. And why it is not a good idea to heat a gas in a closed container. The behavior of gases will be explained in this chapter using the kinetic molecular theory of gases.