Learn that fish live in water and breathe through gills, and observe a fish if possible.
Fish live in water. They cannot breathe air like you do.
Fish breathe through gills, special parts on the sides of their head.
Mastery looks like: They can say that fish live in water and breathe through gills, not lungs like people.
Common stumbles: Thinking fish breathe the exact same way people do is common; the simple idea of gills for water, lungs for air, is enough for now.