The rats and mice are classified in the family Muridae of the suborder Myomorpha. Many a time it is very difficult to differentiate between rat and mouse, as there is no sharp dividing line of these two rodents. Nevertheless, the word "mouse" is loosely applied to almost any of many small rodents usually having rounded ears and long skinny tails. On the other hand, rats are generally larger than mice and most rats are characteristically having scaly tails. Among all the rats, the best known species is the brown or Norway rat (Rattus norvegicus). It is widely distributed in all over Europe and some parts of United States. Rats are devoid of a gall bladder. They are primarily nocturnal. Rats are able to breed at three to four months of age and can produce upto 7 litters a year.