Colleen Hughes, age 11, of Toronto, Ontario, Canada, for her question:

How do artesian and ordinary wells differ?

An ordinary well dips down below the level of the ground water and the water seeps through its porous sides. When you draw up a full bucket, more water seeps in to keep the top of the well water level with the top of the ground water. This is a gentle process. An artesian well gushes to the surface and sometimes squirts a fountain of sparkling water high into the air. It too draws from a supply of ground water. But special rock formations change the method of delivery.

Systems of artesian wells occur where ground water is trapped in porous rocks, capped with layers of dense shale. Often a deep layer of porous sandstone is tilted and percolating ground water seeps down a slope. The solid roof seals the ground water under pressure and prevents it from bursting forth in springs. When wells are dug through this dense top layer, the surrounding pressure spurts the water up to the surface.