The commercial resonance demonstration apparatus shown in Fig. 1 exhibits curious behavior. It consists of three pairs of slender spring-steel rods attached to a horizontal bar. When one of the rods is pulled aside and released, the rod of corresponding length is excited into visible motion, but the other rods remain apparently stationary. This may seem surprising because there is no noticeable motion of the bar or the base. The effect is achieved because each length of rod has its own frequency, and all of the rods are very lightly damped. A very lightly damped rod responds quite strongly when excited at its resonant frequency but barely responds at other frequencies (Fig. 2).