What structural features are common to all mammals?

Features of the structure of mammals:

most have warm blood;
most do not lay eggs;
most do not fly (except for bats);
they have hair or coat (in fact, even whales and dolphins have vibrissae-shaped hair, commonly called whiskers);
they all have lungs.

During reproduction of most mammals, the young grow inside their mother. They say that mammals are viviparous animals. Only a few mammals from Australia, Tasmania, and New Guinea, such as the platypus and the echidna, are oviparous (that is, they lay eggs).