Michell Jolliffe, age 10, of Arlington Heights, Ill., for her question:

WHAT MAKES A GEYSER ERUPT?

A geyser is a hot spring that throws hot water andsteam high into the air with great force.

When an underground water supply is located in an area where the earth holds very hot rocks, the water near the bottom of a long column may reach the boiling point. Held down by cold water above it, the hot water will soon start to steam and form bubbles, pushing some of the water in the channel out of the opening. The column becomes lighter and more steam is formed. Soon the pressure is built up so that there's an explosion of steam and water. Water then seeps back into the channel, and a build up of pressure starts again.

Old Faithful geyser in Yellowstone National Park erupts for about four minutes every 65 minutes. Water shoots up about 150 feet. It hasn't missed its schedule for 80 years at least.