Let's go over to the aquarium and watch fish breathe. Can you think of anything more exciting? When my daughter was three years old, we got some goldfish. She really liked those fish, and the first day we had them, she spent much of that day just looking at the fish. Late that night, almost midnight, she came to my bed and awakened me; she was crying. She said, "Mommy, the fishes' cheeks are coming apart!"

Let's pick up on the largest fish, and watch them closely. We want to count the number of times in one minute that it opens and closes its mouth. Next, we'll count the number of times the flaps (gill covers) on its gills opens and closes--see if his cheeks come apart.

When its gill covers are open, is its mouth open, or is it closed?

I think he can't have them both open at the same time.

What do gills do? What is their purpose? Do they help the fish to breathe? How?

As the fish opens its mouth, water goes in. The water passes over its gills (inside its body), and the oxygen that is dissolved in the water passes through the very thin tissues (like skin) inside, and goes right into the fish's blood-stream. Then it leaves the fish's body through the gill covers. That means, then, that the fish opens its mouth to get water, from which to take in oxygen. When there is enough water inside, it closes its mouth, and opens its gill covers. When the water is almost all out, the fish opens its mouth and starts all over again.

We will boil some water for five minutes, to remove any dissolved oxygen. Put a lid on it, and set it aside to cool. When it is room temperature, we'll put a fish into it. Watch it closely, because we can't leave it in there for very long. There is little or no oxygen in the water, and if it were in there for more than a few minutes, it would die. Would it drown? Or would it suffocate? We'll have to find out from a book.

The information that the fish need oxygen in order to live can be a reminder to us to make sure our aquarium is chemically balanced. If there is rotting food in the aquarium, it has a bad effect on the amount and quality of soluble oxygen available to the fish.