This Demonstration simulates the movement of "atoms" vibrating around a pulsating source. These might, for example, represent radial longitudinal oscillations of sound waves. The source oscillates sinusoidally at a chosen frequency and then movement of the seven circles of atoms follows the pulsation of the blue ball with an increasing phase delay. During one cycle, the pulsation occurs at a chosen frequency (1 to 10 cycles/sec) with increasing and decreasing sinusoidal amplitude. The volume of the blue ball and the position of the outer atoms may be read on the upper graph.