Mammals are animals that have hair and give birth to live young. Although this cannot be correct since the platypus lays eggs instead of giving birth to live young. But then again, the platypus is a monotreme, which may or may not be a mammal - I don't know. Fortunately biologists know that they are mammals. What mammals do have in common is that they all feed their young milk.
It's a common misconception that because all mammals produce milk and have nipples, they could be milked. For example, no one could milk Chuck Norris, nor would it be wise to try. Because Chuck is waiting. For. Exactly. That. Scientists could milk chuck norris. With the proper hormones they could stimulate breast growth and milk production. Also a prostitute can milk Chuck.
Primitive mammals had 11 teeth on each side of the jaw consisting out of 3 incisors, 1 canine, 4 premolars and 3 molars. Compare this to humans with 8 teeth on each side, 2 incisors, 1 canine, 2 premolars and 3 molars. Often the last molar is absent.
Many mammals have modified this pattern even further, often incorporating a diastema region. For instance the mouse has only 1 incisor in each half jaw, no canines, no premolars and 3 molars.