What characteristics are common in all mammals?

There are several characteristics that classify mammals as mammals, here are the main ones:

Breath air

Vertebral Column

Reproduce live offspring

Have hair or fur

Warm-Blooded

Produce milk

(Others):

Single boned jaw

4 chambered hearts

3 bones in middle ear

1 time tooth replacement (at least) - For example, elephants and manatees have "marching molars".

Diaphragms

There is many but some main ones are that we breathe oxygen, were warm blooded, we have hair, give live birth and breastfeed