Charles's law
Leave a slightly soft balloon out in the sun and it swells; pop it in the freezer and it shrivels. Charles's law captures this: for a fixed amount of gas at a fixed pressure, warming it makes it expand and cooling it makes it shrink, in a strict proportion.
Stated exactly, the volume of a gas is directly proportional to its absolute temperature — double the kelvin temperature and the volume doubles. Kinetic theory explains it: heating speeds the molecules up, so to keep the pressure from rising the gas must spread into a larger volume. Plotted against absolute temperature the volume falls along a straight line that, extended, points to zero volume at absolute zero.
Charles's law is another special case folded into the ideal gas law, and it is the principle behind hot-air balloons: heat the air inside, it expands and grows less dense than the cooler air outside, and the balloon rises. It also gives a striking hint of absolute zero, the temperature where the line of volume versus temperature would reach nothing.
A hot-air balloon flies because a burner heats the air inside it; by Charles's law that air expands, becomes lighter than the cooler air around it, and the whole balloon lifts off.
Heat expands the air inside a balloon until it floats — Charles's law aloft.
The proportionality holds only with absolute (kelvin) temperature. In Celsius the relationship is not proportional at all — a gas at 20 degrees Celsius does not have twice the volume of one at 10 degrees Celsius.