Here's why:
* Mammals give birth to live young: Sharks lay eggs or give birth to live young that develop inside their bodies, but they do not nurse their young with milk.
* Mammals have hair or fur: Sharks have scales, not hair or fur.
* Mammals breathe air: Sharks have gills and breathe underwater.
* Mammals are warm-blooded: Sharks are cold-blooded, meaning their body temperature changes with the environment.
Sharks belong to a group called fish which is a very different class of animal than mammals.