A skylight is a type of window or structure found exclusively on ceilings, designed to let more natural light in than a conventional window. They can range in size from only a few square meters in area to occupying the majority of ceiling space. Like normal windows, skylights are usually glazed or covered in some other transparent or translucent material like a float glass and are held in place by frames to prevent collapse. Unlike typical windows, however, most skylights cannot be opened, due to their less than convenient position high above the floor. Those that can open usually can only open just enough to allow for efficient air-flow while keeping out potential rainwater.