Here's why:
* Gills: Sharks breathe underwater using gills, unlike mammals who breathe air with lungs.
* Scales: Sharks have scales, while mammals have fur or hair.
* Cold-blooded: Sharks are cold-blooded, meaning their body temperature changes with the environment. Mammals are warm-blooded and maintain a constant body temperature.
* Reproduction: Sharks lay eggs or give birth to live young, but they don't nurse their offspring like mammals do.