A window screen, insect screen or bug 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, debris, birds, and other animals from entering a building or a screened structure such as a porch, while permitting fresh air-flow. Most houses in Australia, the USA and Canada have screens on all operable windows, which are most useful in areas that have large mosquito populations. Screens in North America were traditionally replaced with glass "storm windows" in cold climates to insulate the window during the winter, but frames combining both storm and screen panels have become the most common type of screen currently used in cold climates.