Because its internals are much like human internals.
There are two things which will tell you if an animal is a mammal or not.1. All mammals make milk to feed their their young. Only mammals do this. Don't be fooled by breasts and nipples. Some mammals...
sharks, birds, fish,ect.-whatever lays eggs
in land, the African elephantmarine, the blue whale
A crocodile is a reptile, not a mammal.