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A window is an opening in a wall or roof of a building, in a car etc., to let air and light in. It is usually filled with a sheet of glass. There can be many different shapes and sizes, including rectangular, square, circular, or irregular shapes. Some windows include coloured glass. Windows are usually transparent so that people can see through them.
Before glass was used in windows, people in Asia used paper to fill the hole in the wall. The paper would let light in.