Rebecca Church, age 7, of High Point, N.C., for her question:

IS MOST WATER PURE?

Water covers about 70 percent of the surface of the earth. Also, about two thirds of the human body is water. So there's certainly a lot of it around.

But, surprisingly, most of the world's water is not pure. Except for rainwater and the water found in the ice fields around the poles, water generally has some impurities in it.

Sea water has lots of salt in the form of sodium chloride. Well water and water from mountain streams have large amounts of iron salts and sulfides. Even spring water has lots of minerals in it.

Harmful impurities are removed by purification plants to make it acceptable for human consumption. One method of water purification calls for the addition of chlorine.