Here's why:
* Kingdom: All animals belong to the Kingdom Animalia.
* Phylum: Mammals belong to the Phylum Chordata.
* Class: Mammals are specifically classified within the Class Mammalia.
* Order, Family, Genus, Species: These categories further classify mammals into specific groups.
So, if an organism is classified within the Class Mammalia, it is a mammal. If it's classified in any other Class, it's not a mammal.