Here's why:
* They breathe underwater using gills. Mammals and reptiles breathe air with lungs.
* They are cold-blooded. This means their body temperature changes with their environment. Mammals and birds are warm-blooded and can maintain a consistent internal temperature.
* They have scales. Most fish have scales, while mammals have hair or fur, and reptiles have scales or plates.
* They lay eggs or give birth to live young. Some sharks lay eggs (oviparous), while others give birth to live young (viviparous). This differs from mammals, who always give birth to live young, and reptiles, who mostly lay eggs.
So, while sharks share some characteristics with mammals and reptiles (like being aquatic), they are classified as fish due to their unique features.