Tides are the regular rise and fall of sea levels caused mainly by the gravitational pull of the moon, and to a much lesser extent by the sun, in conjunction with the rotation of the earth. Although these same forces affect the planet as a whole, the effect on the land masses is much less than that on the oceans and other water bodies because they are less flexible. Tides are conspicuous on ocean shores but are very small and usually not even noticeable on the shores of lakes, even the largest ones.
The existence of tides has resulted in the evolution of a large variety of organisms that are specifically adapted to living in intertidal zones, which are the areas above the water surface when it is at its lowest level (low tide) and below the water surface when it is at its maximum level (high tide). Tides, because of their great power and high degree of regularity, are also showing great promise as a source of renewable energy.