A pulsating source of hot water that jets upward from the ground to make a spectacular boiling fountain.

Old Faithful geyser, Yellowstone National Park.

Geysers are produced when cold water seeps down into rocks that are very hot and stops the hot water from flowing to the surface.

When cold water flows down the same pipe that the heated water is trying to rise in, the cold water acts like a lid, trapping the hot water and making it get hotter and hotter. Eventually the pressure of the superheated water pushes the cold water out of the pipe, shooting it upward as a fountain. The cycle then begins again.

Old Faithful is the most famous geyser in the world. It erupts a fountain of water about once every 50 minutes, hence its name.