Rats and mice are the most common rodents librarians are likely to encounter. Rats are difficult to control because they are capable of gnawing through cinder block, lead and aluminum sheeting, wood, plastic, and sheetrock. The most common rats are the Norway rat (Rattus norvegicus) and the roof rat or black rat (Rattus rattus). The house mouse (Mus musculus) is very common and extremely difficult to eradicate entirely. Both rats and mice use paper to make their nests, and many fine books have lost chunks of text through their jagged gnawing. Rodents' fecal matter and urine are especially damaging. It is generally better to trap rodents than to use a poison that will allow them to crawl into building crevices and die, for rodent carcasses are breeding grounds for insects that also damage library and archival materials.