Lots of animals live in water. But not all of them are fish! A fish has a special body for living under water its whole life.
Key idea
Fish live in water, breathe with gills, swim with fins, and have a backbone. Most fish have scales.
Why it works
Gills are the big clue. Water flows in through a fish's mouth and out past its gills, and the gills take oxygen from the water. A whale has no gills, so it must swim up to breathe air.
Worked example: Let's think it through
A jellyfish has "fish" in its name. Is it a fish?
- Does it have gills? No.
- Does it have fins and a backbone? No.
- A name can fool you. The body tells the truth.
Answer: No. A jellyfish is not a fish.
Common mistake
Children often call every water animal a fish: whales, dolphins, jellyfish, sea stars and crabs. Only animals with gills, fins and a backbone are fish. Sharks are fish!
For the grown-up. At an aquarium, play "fish or not?" at each tank. Look for gills (the slits or flaps behind the head) and fins.