Here's why:
* Gills: Whale sharks breathe through gills, which are the defining characteristic of fish.
* Scales: They have scales, which are also a characteristic of fish.
* Cold-blooded: Whale sharks are cold-blooded, meaning they rely on their environment to regulate their body temperature. This is another trait of fish.
* Lay eggs: While they are viviparous (give birth to live young), they still lay eggs inside their bodies.
While their name might make you think they're mammals, they are actually the largest fish in the ocean!