The box is covered by a thick transparent sheet of glass. When the box is placed in the sunlight, the glass cover allows the infrared rays present in sunlight to pass into the box. Then these rays are absorbed by black surface of the box and the box becomes hot. But, after sometime when the black surface becomes hot it starts radiating out heat in the form of infrared rays. But, the glass sheet cover placed over the box does not allow the heat radiated by the black surface to go out from the box. In this way, the glass cover enables the cooker to entrap the heat inside it.