Worked example - the helium-neon laser's four-level scheme. Step 1 (pump): an electric discharge excites helium atoms to long-lived metastable states. Step 2 (energy transfer): colliding He atoms hand their energy to neon atoms, pumping Ne into its 3s/2s upper laser levels - creating population inversion against the emptier 2p lower levels. Step 3 (laser): stimulated emission at 632.8 nm (red) as Ne drops 3s -> 2p. Step 4 (drain): the 2p level decays fast to 1s, keeping the lower level empty so inversion persists. The mirrors make photons pass repeatedly, cloning more - a continuous red beam from a clever atomic relay race.