Make a pendulum using a string about 10" (25 cm) long, and a mass (a key or bunch of keys works well). Now, hold the end of the string and move it back and forth by an inch or two about three times per second, about once per second, and about once every three seconds. Carefully note the amplitude (how far the mass swings) and the phase (is your hand usually out ahead of the mass pulling it forward, or behind pulling it back). What do you observe?