ELI5: How do fish breathe underwater?

Fish breathe underwater by using their gills to catch tiny bits of invisible oxygen floating in the water, just like your lungs catch oxygen from the air.

Think of it like making a cup of tea. Even though you cannot see it, the tea flavor is mixed all through the hot water. In the very same way, real air—called oxygen—is mixed invisibly into lakes and oceans.

Because humans have lungs, we cannot pull that air out of liquid. But fish have special body parts called gills on the sides of their heads, hidden under little flaps. You can think of gills as super-fine kitchen strainers or sieves.

Here is how fish take a breath in everyday life:

• Taking a sip: The fish opens its mouth and gulps in a big mouthful of water.
• Pushing it through: Instead of swallowing the water down into its tummy, the fish pushes the water sideways and out across its gills.
• Catching the air: As the water rushes through the gill "strainer," tiny blood vessels grab the hidden oxygen and leave the water behind.
• Letting it out: The used water flows out through the slits on the side of the fish's head, and the fresh oxygen travels all through the fish's body to give it energy to swim.