There’s a rule in physics called the law of the conservation of energy. Energy can neither be created out of nothing, nor disappear into nothing — all it does is change forms. That sounds simple, but it confused people for a long time. To see why, let’s think of an easy example, and then a harder one. You roll an eight ball across a billiard table. When it hits the cue ball, the cue ball starts moving and the eight ball stops. Why? Because the kinetic energy you put into the eight ball with your hand has been passed into the cue ball. Notice that no energy is lost, it just traded places. Okay, that was easy. Now suppose you see a car going down the street at a constant speed. Suddenly the driver slams on the brakes. The car comes to a halt. Where did the energy go?