Although whales five in the sea and have a body shape similar to fish, whales are not fish. Fish take oxygen from the water with gills. Fish have scales. Fish are cold-blooded. That is, a fish's temperature is usually about the same temperature as that of the water it swims in.
Fish move their tails side to side to swim.
Whales move their tails up and down. Explain that whales are mammals that live in the sea. Define what a mammal is for your students, using the background information above. Make a list on the board of words that describe whales. Make a second list of words that describe fish.