Life expresses itself in innumerable ways. There are many sorts of animals, and over the course of history they have been classified in several categories defined by certain characteristics.
Most mammals are almost all fur-bearing and give birth to fully developed babies.
They are all nourished with their mothers' milk and require maternal care and attention in the early stages of their life. Birds are feather-bearing and lay eggs. Fishes are scale-bearing and also lay eggs. They all live in either salt water or fresh water. They don't breath air with lungs but rather extract the oxygen in the water through gills.