Rodents are mammals of the order Rodentia, which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents (2,277 species). They are found in vast numbers on all continents except Antarctica. They are the most diversified mammalian order and live in a variety of terrestrial habitats, including human-made environments. Most rodents are small animals with robust bodies, short limbs, and long tails. They use their sharp incisors to gnaw food, excavate burrows, and defend themselves. Most eat seeds or other plant materials, but some have more varied diets. They tend to be social animal and most live among humans or other animals. Mating among rodents can vary from monogamy, to polygyny, to promiscuity. Many have litters of underdeveloped, altricial young, while others are precocial at birth.
The distinguishing feature of the rodents from rats is their pairs of continuously growing, razor-sharp, open-rooted incisors. These incisors have thick layers of enamel on the front and little enamel on the back. Because they do not stop growing, the animal must continue to wear them down so that they do not reach and pierce the skull. Rodents can cause many problems among humans such as destruction of crops, household stuffs and can make living harder. They make surrounding polluted and unhealthy which leads to various diseases as well. Rodents are beneficial in one way but can also make life difficult if the number is very high. Rodents are used as food, for clothing, as pets, and as laboratory animals in research. Some species, like the brown rat, the black rat, are serious pests, eating and spoiling food. House mouses can spread various diseases.