Most mammals use sexual reproduction, where fertilization is internal, and give live birth.
Some mammals are oviparous (egg laying) mammals called monotremes, namely platypuses and echidnas -- the young are not born live but are nursed after they hatch.
Pouched mammals and placental mammals give birth to tiny young ones, so are called viviparous. Sexual intercourse
Bear live young, nourish their young with milk from mammary glands, and usually have hair or fur.
The blue whale is the heaviest living mammal in the water , while the heaviest land mammal is the elephant.
The three toed sloth is the slowest moving land mammal.