How do fish breathe underwater?

Fish breathe by moving water across gills. Water holds a small amount of oxygen, and thin blood-rich surfaces inside the gills take that oxygen into the fish's blood while carbon dioxide leaves the body and drifts away with the flow.

The lovely engineering is in the folds: gill filaments create a huge breathing surface in a very small space. Even a fish hovering almost still may be gently working its mouth and gill covers, keeping fresh water passing over those delicate sheets.