Animals can be classified into groups, according to their characteristics.

Most mammals

are warm-blooded

have hair or fur

give birth to live young

feed their babies with mother's milk

MAMMALS

Most reptiles and amphibians

have lungs for breathing

are cold-blooded

give birth to young hatched from eggs

REPTILES and AMPHIBIANS

Reptiles and amphibians are "cold-blooded." This doesn't mean they have cold blood, but that they don't produce their own heat like mammals. They need to get heat from the sun to warm themselves.

So what's the difference between reptiles and amphibians?

have dry, rough, scaly skin

lay their eggs on or under dry ground

begin their life cycle on land

have moist, smooth skin without scales

lay eggs in water or in damp areas

begin their life cycle in water and then, as adults, their bodies change to adapt to life on dry land

Most birds

have two wings, two legs and a beak

are covered with feathers

are warm-blooded

give birth to babies hatched from eggs

BIRDS

Most insects and arachnids

- have hard outer shells called exoskeletons

- have multiple pairs of legs

- breathe through holes in the sides of their bodies

- are cold-blooded

- give birth to young hatched from eggs

INSECTS and ARACHNIDS

Most fish

live in water all their lives

breathe oxygen from water through special organs called gills

have fins and scales

are cold-blooded

give birth to babies hatched from eggs

FISH