Waves carry energy. A wave is only formed if there is a disturbance. When a stone falls into a pond, some of the kinetic energy possessed by the stone is transferred to the water. This causes the water to move up and down, and we see waves spreading out from the point of impact. If there is a leaf floating on the pond it moves up and down as the wave passes it. The leaf receives some of the kinetic energy that came from the stone. The water which carries the wave just moves up and down, it is not carried along. Waves carry energy without transferring matter. Light and sound both travel as waves. The speed of the wave depends on the substance that it is travelling through.