Carla Phillips, age 7, of Glasgow, West Virginia, for her question:

Why can't all birds fly?

There are about 8,600 species of birds in the world    and most of them can fly. But a few of them can't fly any better than we can. One of these is the ostrich, who happens to be the largest bird in the world. He can never get off the ground    but he is a very fast runner. The handsome penguin cannot fly either    but in the water he is a champion swimmer and diver.

The proud ostrich stands eight feet tall, and weighs about 300 pounds. He can race along at 35 miles an hour, much faster than any man can run. But, as everybody knows, the ostrich cannot fly like most birds. Perhaps he is just too heavy to lift his body off the ground. This has something to do with it, but there are other reasons why the ostrich and certain other birds cannot fly.

To be able to fly, a bird must have a very special body structure. He must be strong and very light with hollow bones. Inside his body there must be empty pockets called air sacs, which fill up with air to make him lighter. His wings must have extra strong,, stiff feathers with similar tail feathers. And a bird needs one more thing to take him up into the air    very strong chest muscles to power his wings.

His body needs a very special bone to fix and hold his extra strong "flying muscles" in place. This bone is called the keel because it is shaped somewhat like the curved keel at the front of a ship. It is located at the front of his chest, and it is commonly known to us as the wishbone.

The ostrich wears huge fluffy plumes but has no stiff flying feathers on his wings or tail. His bones and his thick leg muscles are too heavy for him to lift himself with his weak wings. But most important, he has no keel bone in his breast. Without this he cannot have the chest muscles that every bird needs to fly. The ostrich's chest bones are shaped like a raft. An older word for raft was used to name birds that have such chest bones. They are called ratite birds. Most of the ratites are large birds, although not quite as large as the ostrich. One is the emu (five feet tall) who lives in Australia. The big bad tempered cassowary also lives in Australia. Both of these birds are good swimmers. The long legged rhea is a smaller ostrich cousin who lives in South America. None of these birds can fly, but all of them are excellent runners.

Some birds cannot fly, even though they have keel bones and plenty of chest muscles. One of them is the cheerful black and white penguin. His wins are too small for flying, so instead, he uses them as flippers to swim and dive in the water.

Some birds must give up flying because they get too heavy for their wins. This is why fat chickens are poor fliers. The chicken sized kiwi bird of New Zealand has very tiny wings. But he is a ratite bird    so he could not fly, even if his wings were bigger. '