Rodents
The largest family of mammals is Rodentia, or the rodents. Rodent means gnawing animal, and these animals are so named because of their large incisor teeth
and the way they eat. The two long pairs of incisors are used like chisels
to gnaw on hard foods like nuts and wood. Their incisors  grow continuously
and must be worn down by gnawing.
There are three major types of rodent: squirrels, mice,
and porcupines. 
  - Squirrel-like rodents, such as the squirrel and gopher, have long, bushy 
    tails and large eyes. They can live in trees or underground in tunnels. They 
    might hibernate during the winter. 
- Mouse-like rodents include the mouse, the rat, and the hamster. Some mouse-like 
    rodents have long, thin tails and short legs. Others have short tails. They 
    live mostly above ground, although some burrow underground. They might also 
    hibernate during the winter. Rats and mice often live near humans, sometimes 
    even inside humans' buildings, enabling them to live off human food and garbage. 
  
- Porcupines differ from other rodents, and other mammals as well, because 
    they have long, sharp quills on their backs for protection. 
