Rats are usually distinguished from mice by their size. In general, when someone spots a large-sized rodent, the common name includes the term rat, while if it is smaller, the name includes the term mouse. The rat family is extensive and complex, and the common terms rat and mouse are not taxonomically specific. Scientifically, the terms are not limited to members of the genus Rattus and Mus, for example, the pack rat and the cotton rat.