Just like humans, fish need to breathe to survive. And just like us, they also need to breathe in a gas called oxygen and breathe out a gas called carbon dioxide. However, they live in a very different environment from us - underwater. We can't breathe underwater, so how do they manage?

Well, to survive, fish need to get oxygen out of the water and into their blood. They do this using gills, small flaps of skin on the side of a fish just behind their head - you may have noticed them if you've ever looked at a fish.

Firstly a fish will open its mouth to let in water. By lowering its bottom jaw, the fish lets the water flow back to its gills. It then closes its mouth, and the gills remove the oxygen the fish needs from the water, while passing the carbon dioxide it doesn't need from its blood back into the water. The gills then open, letting the water leave. This process is repeated constantly, and is the fish equivalent to a human breathing in, then out.

Getting oxygen out of water is much more difficult than getting it out of air. Water is 1000 times heavier than air and while 21% of air is made up of oxygen, less than 1% of water is oxygen.

So you can see that getting oxygen from water isn't easy. We simply breathe in air and 21% of it is oxygen. Fish have to use much more energy getting theirs, dealing with water - 1000 times heavier than air - to get less than 1% oxygen. If we had to use so much energy to get so little oxygen, we wouldn't live for very long.

Fortunately, fish are very good at extracting oxygen from water. While humans only extract about 25% of the air that comes into our lungs, fish are able to get 80% of the oxygen in water.