* They don't give birth to live young. Sharks are oviparous (egg-laying) or viviparous (giving birth to live young, but without a placenta).
* They don't have mammary glands. Mammals are named for their mammary glands, which produce milk to nourish their young.
* They don't have hair or fur. While some sharks have small, tooth-like scales, they don't have hair or fur like mammals.
* They breathe through gills. Mammals breathe through lungs, while sharks use gills to extract oxygen from the water.
* They are cold-blooded. Mammals are warm-blooded, able to regulate their own body temperature. Sharks are cold-blooded, meaning their body temperature is determined by the environment.
Sharks belong to the class Chondrichthyes, which includes other fish like rays and skates. They are more closely related to these animals than to mammals.