How do BB guns work?

Howstuffworks: "The principle behind BB guns is pretty much the same as in firearms, except there is no explosion. The expanding matter is ordinary air, carbon dioxide or some other gas. Before you fire the gun, the gas is compressed so that it has a greater density, and consequently a greater pressure than the air in the atmosphere outside the gun. The compressed gas is stored in an airtight container until you pull the trigger. This opens up the gas container so the gas can flow out into the barrel, just behind the BB. Since it is more compressed, the gas behind the BB pushes out with greater force than the air in the atmosphere pushes in, and the BB is propelled forward at great speed. This is the idea behind all gas-powered guns, including high-powered air rifles and paintball markers. "