Mammals have hair, warm blood and feed milk to their young.
There are mammals on land, in water and on air. Bats are mammals because they have hair, warm blood and feed milk to their young. Bats have wings but unlike birds, bats have 5 fingers. 4 fingers developed wings and the other finger is used for holding on to walls.
There are marine mammals too like Dolphins for example. Dolphins are warm blooded and feed milk to their young.
Most mammals live on land like us humans. Mammals blood always have the same temperature whether its cold or hot.
Hair is what keeps us warm during the winter.
Mammals give birth to babies when its fully developed inside them.