Mammals are the most popular class and they are vertebrates. They have a backbone,  which is why they can move with great ease. They all have lungs and breathe in air. In addition, they are warm-blooded, which means that they are able to maintain a constant body temperature regardless of the outside temperature. Mammals are the only animals that possess true hair and they have the capacity to produce milk. Most of them walk on four legs.

The word mammal comes from the latin word "mamma", which means "breast". Normally mammals don't produce eggs, but there are two types that lay eggs: the "platypus" and the "echidna".

The current extinction rate is 130 species per day. The organisation WWF works against the extincion of species. There are between 3,500 and 5,000 species of mammals today.