What do you suppose would happen if we continuously applied energy to the elevator? Electrons would ride the elevator one after the other directly to the fifth floor (excited state) and then go down to the fourth floor (metastable state) where they would accumulate. This results in the phenomenon called population inversion, in which more electrons end up on the fourth floor (metastable state) than on the first floor (ground state).

However, since the fourth floor (metastable state) is not completely stable, one electron in the population inversion goes down to the first floor (ground state), releasing energy in the process and causing others to surge back to the first floor. This kind of energy release is called "stimulated emission," and this phenomenon is the means through which laser beams are produced.