This document describes the key characteristics of 5 different animal groups: Mammals have hair or fur, give birth to live young, feed babies with mother's milk, have baby teeth that fall out before adult teeth come in, and are warm-blooded. Fish live in water, lay eggs, have scales, fins, and breathe using gills. Birds can mostly fly, have wings and feathers, lay eggs, have hollow bones, and are warm-blooded. Amphibians can live both on land and in water, have permeable skin, breathe using gills as young, and have moist skin. Reptiles have scales, breathe with lungs, most lay eggs, have dry skin, and are mostly