Jeri Peters, age 9, of Hawarden, Iowa, for her question:

Why do lakes turn green in summer?

Water turns green when it teems with tiny plants called algae. Most of them are too small to be counted one by one. But when millions of them are floating in the water, they add a greenish tinge. Like most plants, algae use sunlight to make their food. So naturally they make the most food during the long days of summer. When the tiny plants are well fed, they multiply very fast. Suddenly the lake water turns green. In winter, their numbers dwindle down to just a few. The lake water is clear until next summer returns with long hours of warm sunshine.

Tadpoles, baby fishes and many other small water creatures feed on floating algae. But sometimes these tiny plants get tangled in the gills of larger fishes and suffocate them. These fishes need to keep their gills clean to take in oxygen. Many of them die when the lake water gets clogged with too many little green algae.