Unless it has a temperature of absolute zero (-273°C) an object reflects, absorbs, and emits energy in a unique way, and at all times. This energy, called electromagnetic radiation, is emitted in waves that are able to transmit energy from one place to another. For example, this computer, trees, air, the Sun, the Earth, and all the stars and planets are reflecting and emitting a wide range of electromagnetic waves. These waves originate from billions of vibrating electrons, atoms, and molecules, which emit and absorb electromagnetic radiation in unique combinations of wavelengths.
The amount of electromagnetic radiation an object emits depends primarily on its temperature. The higher the temperature of an object, the faster its electrons vibrate and the shorter its peak wavelength of emitted radiation. Conversely, the lower the temperature of an object, the slower its electrons vibrate, and the longer its peak wavelength of emitted radiation. This concept can be shown by gripping the end of a long rope and shaking it. Rapidly shaking the rope (high temperature) results in a series of short waves travelling along it, while shaking it slowly (low temperature) results in a series of longer waves.