In a given container, billions of particles of gas are constantly moving around randomly, colliding with the sides of the container. With thousands of collisions per second, individual collisions do not register, but an average that amounts to a constant force known as the pressure of gas.
When air is blown into a balloon, the balloon expands because the amount of gas increases, increasing the number of gas particle collisions, thus increasing the gas pressure inside. Pressure determines the direction in which mass flows, so if the balloon is released, the gas inside moves from a high-pressure area to a low-pressure area.
There is a roughly constant atmospheric pressure on Earth that equals the weight of about 10 meters of water. This pressure normally goes unnoticed because it is pushing in all directions at once. The altitude of an object affects the amount of pressure exerted on it. For example, at 20,000 feet in the air, atmospheric pressure equals about one-half that at sea level. This is because about half the entire atmosphere is below this altitude.
One unit of measure of pressure is pounds per square inch, and atmospheric pressure at sea level is 14.7 pounds per square inch. The pressure of car tires is also measured with the same unit. The standard car tire requires 26 to 30 pounds per square inch of pressure.