William Shockley, Nobel-Prize winning co-inventor of the transistor (a revolutionary electronic amplifier dating from the 1940s) had a vivid way of explaining it: "If you take a bale of hay and tie it to the tail of a mule and then strike a match and set the bale of hay on fire, and if you then compare the energy expended shortly thereafter by the mule with the energy expended by yourself in the striking of the match, you will understand the concept of amplification."
