A window screen, insect screen, or fly screen is a metal wire, fiberglass, or other synthetic fiber mesh, stretched in a frame of wood or metal, designed to cover the opening of an open window. Its primary purpose is to keep insects, leaves, birds, and animals from entering a building or a screened structure such as a porch, while permitting fresh air-flow. Most houses in North America have screens on all operable windows, which are most useful in areas that have large mosquito populations.