High on a snow-capped mountain, three drops of water begin their journey toward the sea. From a clear cold mountain pool, they pass into a stream that becomes a river. The story science teacher Sigmund Kalina has to tell about the ecology of the river, how it is affected by pollution and how it renews itself, is as fascinating as fiction. Mr. Kalina is the author of The House That Nature Built, about the pioneer plants that turned the earth into a garden. His Air, the Invisible Ocean, told about earth's atmosphere. Now, in Three Drops of Water, he tells the story of water, without which there would be no life. His absorbing account, perfectly complemented by the illustrations of Charles Robinson, will awaken even the city dweller to a new appreciation of nature and a better understanding of the interdependence of living things that is ecology.