How Fish Breathe Underwater

A close up of a fish

Ever wonder how fish breathe underwater? Try out the experiment below to learn just how they do it!

Did You Know:

Fish don’t have lungs like you or I. They have specialised organs called “gills” instead.

Did You Know:

There is lots of oxygen in water, whether it’s fresh water (like an inland lake) or saline (salt) water (like the ocean).

Did You Know:

Each water molecule (also known as by its chemical compound name “H2O”) is made up of two hydrogen atoms bonded to one oxygen atom.

Let's Make Our Own Fish Gill and Learn How it Works!


  • Glass jar
  • Cup
  • Spoon
  • Water
  • Coffee filter
  • Coffee grounds
  • Rubber elastic


  1. Fill the cup with water, add a spoonful of coffee grounds and give it a stir. The water in this mixture represents the ocean water, while the coffee grounds represent oxygen.

  2. Spread a coffee filter over the top of the glass jar and secure it with the rubber band. This coffee filter will act like a fish’s gills.

  3. Carefully pour the mixture over the coffee filter (Note: It might be best to do this over a tray or sink to prevent any mess!).

  4. Watch what happens as the coffee (oxygen) and water is poured through the filter. The water should pass through the filter (the gills), while the coffee grounds (oxygen) gets trapped and left behind. 

    This is how a fish breathe underwater: by separating the oxygen from the water with their gills!

The STEM Behind the Fun

How Fish Gills Work

Like our experiment just showed, fish take their oxygen supply from the water using their gills! 

Their gills are specialised organs that filter the water, and pull the oxygen out. Afterwards, this oxygen is absorbed into their bloodstream, where it can be carried all around the fish’s body.


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