Here's why:
* Mammals have fur or hair, give birth to live young, nurse their young with milk, and are warm-blooded. Cats fit all of these criteria.
* Reptiles have scales, lay eggs, and are cold-blooded.
* Amphibians have smooth, moist skin, lay eggs in water, and can live both in water and on land.
Cats don't share these characteristics with reptiles or amphibians.