Many people have misunderstandings about the concept of fish. When they see an animal that lives in water and looks like a fish or has the word "fish" in its name, they think it belongs to a fish. For example, whales (mammals), cuttlefish (mollusks), abalone (mollusks), soft-shelled turtle (reptiles), giant salamander (amphibians), crocodile (reptiles) and so on are often wronged by this misunderstanding. Generally speaking, an animal must meet four conditions to be considered a fish:

First, it must be a vertebrate that lives in water all its life; Some fish can stay on land for a short time.

Second, swim (exercise) through the swing of tail and trunk and the coordination of fins.

Third, breathing mainly depends on the gills.

Fourth, the body surface is often covered with scales.

Sao nian, let me give you a concept. Use your head.