Here's why:
* Breathing: Dolphins breathe air through lungs, just like humans. Fish have gills and breathe underwater.
* Warm-blooded: Dolphins are warm-blooded, meaning they regulate their own body temperature. Fish are cold-blooded, meaning their body temperature changes with the environment.
* Live birth and milk: Dolphins give birth to live young and nurse them with milk. Fish lay eggs.
* Fins vs. Flippers: Dolphins have flippers, which are modified forelimbs. Fish have fins, which are different structures.
Even though dolphins live in the ocean and have streamlined bodies, they are more closely related to land mammals than to fish.