A laser creates light by special actions involving a material called an "optical gain medium". Energy is put into this material using an 'energy pump'. This can be electricity, another light source, or some other source of energy. The energy makes the material go into what is called an excited state. This means the electrons in the material have extra energy, and after a bit of time they will lose that energy. When they lose the energy they will release a photon (a particle of light). The type of optical gain medium used will change what color (wavelength) will be produced. Releasing photons is the "stimulated emission of radiation" part of laser.