When fish live in a tank, air must be driven into the water to supply what the fish use out of it. If air is not driven in, the fish will die. As the oxygen in the water becomes scarce, the fish seem uneasy. They come up to the top of the water, and put their mouths out for air. They give a gasp as if choking. In such tanks, where fish live, a jet of water is driven with some force into the water of the tank from above. As it goes down, you see a stream of shining bubbles. These bubbles are air. They break in the water, and the air mingles with the water in the tank. You may see gold-fish in globes coming to the top of the water, and seeming uneasy and unhappy. Then if you dash in a little fresh water, or take some of the water from the globe and pour it back from a little height, the fish seem content. You know that the driving winds and the tumbling waves that break in foam will keep plenty of fresh air in ocean water, for all the finny tribes that live there. So rivers, as they rush and ripple, toss their waves, and are driven with winds, get air bubbles mixed in their waters. Some fish need more air than others, and often come to the surface to breathe. Some, as the sturgeon, leap above the surface into the air. Perhaps you have seen cat-fish jumping in this way. Did you ever see dolphins and porpoises[31] roll themselves quite above the surface of the water in their play? I have told you of the fish that make short flights. In all these ways these creatures help themselves to air in their water home. But what do they do for food? As I told you, fish eat fish and insects and crabs and such living things. Some few are feeders on vegetables. Fish will also devour almost anything that is thrown into the water for them. Did you ever feed fish in a pond with bread and worms? Carp will hurry to the top of the water to get food that they have learned to look for when people come near. Fish of a kind often keep together and feed together. You know in a stream there may be some places where you will catch only trout, or only bullheads. And in some other place you will find pickerel. In the sea you will find millions of herring together at certain seasons. This is called a shoal of herring. Fish move from place to place along many miles of sea-coast or river, as they seek their especial food, or a certain warmth. Sometimes a particular fish, or pair of fishes, will live for years in some certain place. They may find a nice spot under a bank, where they feel safe, and where plenty of food drifts near them. I knew of a fish that went into a hole made by the roots of a great willow-tree. The roots formed a close cage about him. He liked it well. His enemies could not get at him. Water rippled in and out about him. Grubs and worms were close to his mouth. He grew fat and big. He grew so big he could not get out. He began to grow to fit the shape of his cage. As he had no room to grow long, he grew broad.