Fish are cold-blooded animals that live in water and breathe through gills instead of lungs. Most fish have scales covering their bodies, fins for swimming, and a streamlined shape that helps them glide through water. A goldfish in a bowl, a trout in a mountain stream, and a massive tuna in the ocean are all fish, even though they look very different from each other.

Fish have existed for over 500 million years, making them much older than dinosaurs. They've adapted to almost every aquatic environment on Earth, from shallow tropical reefs to the deepest, darkest parts of the ocean. Some fish, like salmon, even travel between fresh water and salt water during their lives.

The word fish can also be used as a verb meaning to try to catch fish, usually with a rod and line or a net. When you go fishing at a lake, you're trying to catch fish. People also use fishing figuratively when someone asks indirect questions hoping to get specific information: if your friend keeps mentioning her birthday, she might be fishing for hints about what present you got her.

Interestingly, scientists don't actually consider “fish” a precise scientific category, because the group includes species less related to each other than some are to land animals. But in everyday language, we use fish to mean any of those swimming, gill-breathing creatures we find in water.