Courses: P3, P13, P15, P19
Two tuning forks mounted on resonant boxes, differing by about 1 Hz. Place them face-to-face, sound one and let it play for a few seconds, then damp it, and the second one continues sounding. Place a small piece of sticky wax on either one and repeat -- the second one will not be energized by the first.
One can also hang a ping pong ball from a string, such that it rests in contact with the second tuning fork. When that fork starts vibrating the ping pong ball will begin to bounce on and off the tuning fork.
