Kalina Rule, age 10, of Spokane, Wash., for her question:

WHAT EXACTLY IS A FOOD CHAIN?

In the hungry sea, big fishes eat small fishes, small fishes eat smaller fishes  and so on down the line. This is part of a food chain that supplies groceries to a lot of different animals. But sooner or later we begin to wonder what the very smallest fishes feed upon. Actually they feed on floating plankton. This is a sort of seafood salad of tiny plants and animals, most too small for our eyes to see. The plankton is the beginning of the great food chain that feeds all the creatures in the sea.

There are many such food chains on land. For example, the rabbits feed on grasses. Some of them are eaten by foxes, eagles or coyotes. Some of the foxes are eaten by mountain lions. In a food chain, most animals feed on plants and provide meat for other animals. The basic idea may seem cruel  but if the meat eaters did not help to reduce the plant eating populations, there would be no plants left in the world.