All mammals have hair, including marine mammals such as whales and dolphins. Hair (or fur) is one of the characteristics that define a mammal. The fur is used to insulate against the cold, but whiskers are used as sensory organs, especially in nocturnal animals.

Amphibians are a group of vertebrates that has adapted to live in both water and on land. Amphibian larvae are born and live in water, and they breathe using gills.

Amphibians and reptiles are vertebrates—animals with backbones. The majority of species have four legs, but there are a lot of exceptions in both groups. Amphibians are cold-blooded animals with moist skin. Frogs and toads are amphibians. Amphibians do not have hair, feathers, or claws.

Birds are a group of warm-blooded vertebrates constituting the class Aves characterized by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton.

A mammals best method of cooling down is sweating. Sweat is made mostly of water with some potassium, salt, and other minerals. As sweat evaporates from the skin, it carries heat away from the body and allows cool air to rush in which helps reduce your overall body temperature.

There are some mammals, including a group called marsupials, that do not have a placenta. Marsupial babies are underdeveloped when they are born and are usually carried by their mother in a pouch. Other mammals, such as the platypus, do not give birth to live young, but instead lay eggs.

Turtles are reptiles because they are four-legged vertebrates with a cold-blooded metabolism and scales covering their body.

All mammals have fur or hair, give birth to live young, have lungs and need air to breathe, are warm blooded, have four limbs if they live on land, and feed their babies milk from their breasts. We call this nursing their young.

Mammals have large brains for their body size--larger than most members of other vertebrate groups. Just as mammals come in all sizes, so do their brains. But it's the size of the brain relative to the size of an animal's body that really matters.

Birds have nine air sacs: two cervical, an unpaired clavicular, two cranial thoracic, two caudal thoracic, and two abdominal air sacs.