Earth is composed mainly of bodies of water---about 71 percent of the earth's surface is water. Ninety-seven percent of that is found in the oceans. Because the earth's surface is curved, the sun heats it unevenly. This causes winds to blow across the water and generate waves. The sun and moon also exert their gravitational pull on the earth, triggering wave-producing tides. Wave motion energy can be harnessed and generated into electricity.