John McLean, age 10, of Garden Grove, California, for her question:

How can a goldfish move up and down in his bowl?

Goldie, of course, can swim around under water. He can rise up or sink to lower levels as he chooses. A breezy bird can fly around through the air. He can soar up higher or swoop down lower as he chooses. We human beings must spend most of our time with our feet on the ground    while some of the animals seem to be having lots more fun.

One way to keep from sinking in the water is to swim    and, indeed, this is how a goldfish in a bowl spends a good deal of his time. But sometimes he pauses and hovers in one spot, merely waving his gauzy fins. This is when you would expect him to sink to the bottom. And he would sink    if he did not have a special balloon built into his body. This useful balloon is called his swim bladder, or his gas bladder or his air sac.

The flesh and bones of a fish's body are heavier than water. And, as everybody knows, things that are heavier than water sink to the bottom. Things that are lighter than water float to the top and bob on the waves. But some things are so light that they rise right up above the water. These things are gases    like the invisible gases of the air. Although fragments of sir may get trapped and mixed with water, most of the earth's vast blanket of air rises above the land and sea. It is unsinkable.

Air is so light that it can help to lift solid objects up through the water. This is what the fish's air sac does for him. When filled with air, it lifts his body of flesh and bones up to a higher level. When there is less gas in his air sac, he has less lifting power. He sinks down to a lower level. This trick of rising up and down is called buoyancy. When his sac is filled with air, Goldie is more buoyant and he rises upstairs in his bowl.

A fish's air sac is a long, slim balloon inside his body under his backbone. It is made of very thin skin that is dotted with lots of very special cells. These cells can sift the gases that are dissolved in the fish's blood stream and put them into the air sac. They also can remove gases from the air sac and put them back into the blood stream. A fish needs oxygen, just as you do. And just as you do, he totes oxygen and carbon dioxide gases around in his blood stream. Most of the gas used to  fill the air sac is oxygen. When Goldie wishes to swim around the basement of his bowl, he lets some of the gas out of his air sac. With less lifting gas, his body grows heavier and less buoyant. He sinks to a lower level.

Many fishes have air sacs of some sort. A few use them as lungs to gulp air and these fishes can live out of water for a while. Many tiny fishes use their air bladders to make sounds. Sea going scientists are trying to find out just how they do this. Flounders and flatfish who live on the bottom do not need buoyancy to lift them. So they have no air sacs at all. Mackerel, tuna and several other fish have tiny air sacs or none at all. These fellows must keep swimming or they sink to the bottom.