New York City now offers a three-hour cram course on rats, including useful information about how to keep them out of household garbage cans. The predominant breed of city rodent is the Norwegian rat, which, at one pound, can appear larger when it tries to scare off predators, and has teeth capable of gnawing through wood and light concrete. The rats only need a quarter inch hole to get into a home. Participants in the free course, which can include building super-intendants and janitors, get rat-school diplomas and rat-proof garbage cans. For more information on the Health Department’s programs, go to