Hot air balloons, internal combustion engines and the cook timing plunger in a frozen turkey are practical applications of Charles's law. Charles's law states that the volume of a gas increases as the temperature increases if the pressure remains constant, and the inverse is also true.
Jacques Charles developed his gas expansion theory on June 5, 1783, while watching the Montgolfier brothers inflate a balloon with fire, and fly it a mile and a half before returning to the ground. By heating the air inside of the balloon, it expanded but remained constricted inside of the balloon. The air expanded until its volume was greater than the equivalent volume of surrounding cold air, and the balloon began to rise. When the air inside of the balloon cooled to the same volume as the surrounding air, it returned to the ground.
The principle is the same for a car's combustion engine. When the fuel contained in the engine's cylinders combusts, it expands from the heat forcing the piston out of the cylinder. The kinetic energy generated turns the crankshaft and moves the automobile.
The pop-up timer on a frozen turkey purchased at the grocery store follows the same principle. When the internal temperature of the turkey is high enough, the air trapped inside expands and forces the timer to pop up, indicating the bird is cooked.