A ball floats on the sea as a wave passes under it.

A wave is made by energy passing through water. As the energy passes, it makes the water move round in circles.

The most common waves are made by the wind. As wind blows across the surface of the sea or of a lake, the friction between the moving air and the water keeps disturbing the surface and builds up a crest. Storms such as hurricanes, with their very strong winds and changes in pressure, can create dangerous waves, including the long waves of a storm surge, which grow as they near the shore.

Other waves have other causes. Earthquakes, landslides and volcanic eruptions under the sea can suddenly push aside huge amounts of water, setting off a tsunami, a series of very long waves. The pull of the Sun's and the Moon's gravity on the Earth creates waves too: the tides. Although people sometimes call tsunamis "tidal waves", the two have nothing to do with each other.

Perhaps the most surprising fact about waves is that the water itself does not travel with them. A wave carries energy, not water, across the ocean, and a wave that meets nothing in its way can cross a whole ocean basin.