Some plants photosynthesize at night

Plants use the energy of the sun to make food through the process of photosynthesis, right? So surely photosynthesis can only happen when the sun is out? Actually, many desert plants use a special form of photosynthesis known as Crassulacean acid metabolism (CAM). Such plants still collect energy from the sun during the day to fuel photosynthesis. However, gas exchange, i.e. the entry of CO2, needed for photosynthesis, only happens at night, allowing such plants to be much more water efficient.