What sign indicates that a person is a mammal?

The first of the proposed signs is not quite what is needed in order to reliably attribute a person to such a group of the animal world as mammals. After all, turtles, frogs, snakes, newts, crocodiles, birds are not mammals, but breathe with lungs. But, such a sign, how exactly – the birth of cubs, that is, when an animal or, as we know, a person, gives birth and not eggs and eggs are born, but live animals or a human child, this is such a sign that indicates that man is a mammal. And, finally, the third sign – feeding with milk, as a matter of course, because living creatures are born that need to eat something, is also the very sign, which indicates that a person is a mammal.