There are over 4,000 different types of mammals in the world that can all be identified by their unique differences. The blue whale, which is the largest mammal on the earth weighing as much as 1500 men, is obviously different than a bumblebee bat, the smallest mammal weighing less than a penny. Dogs don't look like cats and mice aren't elephants.
All animals have hair or fur.
Even whales have a few course hairs around their mouths. Once baby mammals are born, their mothers feed them with milk from special glands in their bodies called mammary glands. This milk nourishes them until they are mature enough to eat solid foods. Most of these common characteristics shared by mammals may seem familiar to you. We also give birth to live young and feed them with milk.