Nearly all substances contain small pores that allow passage of molecules both in and out. A balloon is no exception. What many people do not realize is that lighter weight gases move faster than heavier gases. When a balloon is filled with helium and placed in a room full of air (composed mostly of the heavier gases nitrogen and oxygen), a competition among the gases is involved. The lighter helium gas molecules leave the balloon faster than the heavier nitrogen and oxygen molecules can enter the balloon. Over a period of time, the balloon shrinks! Odd as it may seem, had the balloon been filled with a gas heavier than the gases in air (argon, for example), the balloon would eventually expand. The molecules in the air will enter the balloon faster than the heavy argon can exit.