Here's why:
* They give birth to live young: Reptiles lay eggs, while mammals give birth to live young.
* They have fur: Armadillos have a bony shell, but they also have hair. Reptiles have scales.
* They nurse their young: Mammals produce milk to feed their offspring.
* They are warm-blooded: Mammals regulate their body temperature internally, while reptiles are cold-blooded and rely on external sources of heat.