Ronald Clark, age 9, of Deer Park, Washington, for his question:

How can the fish stay under water?

A fish never never has to come up for a breath of air. He was born under water and he lives under water, day and night, all through his life. In fact, he has to stay down there in his wet world. And you must stay up here in your dry world. This is because you breathe your oxygen from the air and a fish breathes his oxygen from the water. Your lungs can sift oxygen from the air you breathe in    but they cannot take oxygen that is dissolved in water.

Instead of lungs, a fish has a pair of gills. They are made of feathery threads and placed where you would expect his ears to be. Each gill has a round cover, with an opening at the back. There are tubes inside to guide streams of water from his mouth, through the gills and outside again through the half open gill covers. There are atoms of oxygen dissolved in the water. The gills take them, just as your lungs take oxygen atoms from the air. But gills cannot work in the dry air and lungs cannot work in wet water.