Larry Miller, aged 13, of Elyria, Ohio, for his question:

How does a. fish extract oxygen from the water?

A fish uses his gills much as you use your lungs. Your lungs are a spongey mass of cells, air pockets and blood vessels. The walls of the blood vessels are very thin. The blood in them comes very close to the air in the tiny tunnels and pockets. The blood can draw the oxygen right through the cell walls.

The fish’s gills are where you would expect his ears to be. They are a mass of fringes below a strong round cap. He swallows water in the same manner that you breathe air. The water is sent flowing through his gills as air is sent flowing through your lungs.

His blood vessels too, can reach through cell walls and grab the oxygen he needs to keep going. However, the oxygen you breathe from the air is not blended with any other gas. There are other gases in the air  to be sure but , though mixed together each gas is free and on its own. Hydrogen and oxygen combine to dorm molecules of water. The fish cannot get his oxygen by breaking these molecules apart, He depends on free oxygen in the water.

This free oxygen gets into the water as air and water mix. This happens in waterfalls, tides, dashing waves wind water currents. There are always more fish where the water is turbulent. For there is more oxygen there for them to breathe.