How are baby mammals different from reptiles?

mammal are born already alive and reptiles are hatched form eggs like birds

ANS2:

While some reptiles are born live (garter snakes) instead of being hatched from eggs and some mammals (platypus and echidna) are hatched from eggs instead of being born live, the primary distinguishing characteristic is that mammalian young count on getting milk from the mother. No extant reptile provides milk to her young.