Microwaves are electromagnetic waves whose wavelengths range between about 0.0001 and 0.01 meters. This kind of radiation can be used to heat water, such as in microwave ovens, or for wireless communications, such as in cellular telephones. Microwaves are also emitted by the universe itself. The residual heat from the beginning of the universe leaves deep space at a temperature of about 2.7 kelvin (2.7 degrees above absolute zero), and produces the cosmic microwave background.