Today I found out how fish gills work.
These fantastic little organs allow the fish to absorb oxygen from the water and use it for energy. Functionally, gills are not that dissimilar to the lungs in humans and other mammals. The main difference is how they are able to absorb much smaller concentrations of available oxygen, while allowing the fish to maintain an appropriate level of Sodium Chloride (salt) in their bloodstream.
LINK (via: Today I Found Out)