Valves supply the fuel-air mix into the cylinders and remove the spent gases. The stems of sports valves are hollow and filled with sodium. When exposed to a high temperature, sodium melts, and the speed at which a valve changes the direction of its linear motion pushes the sodium toward its upper part, where it releases heat. Next, the sodium is flung inside the hot valve head, where it melts, taking away the heat and releasing it at the upper part of the stem.