The behavior of gases can be examined by looking at Charles's gas law, which says the volume of a fixed mass of gas at a constant pressure varies directly with its absolute temperature.
This means that as the temperature of a gas is lowered, the molecules move closer together and the volume of the gas decreases proportionally to the drop in temperature. Think about blowing up a balloon indoors and then taking it outside on a cold day. The balloon would get smaller, because the air (gas) inside would shrink in volume. What would happen if you brought the balloon back inside?