Tides are the rhythmic rise and fall of the ocean's surface with respect to its shorelines. Caused by the complex movements of the moon around the earth and of the earth around the sun, tides are also affected by the shapes of the coasts and the sea floors. Tides mix ocean waters, flushing out pollution and bringing oxygen and nutrients to marine life. Most coastal areas have two low tides and two high tides per day. However, some regions have only one set, and enclosed seas like the Mediterranean have almost no tides at all.