Pour salt onto a vibrating plate, change the frequency, and watch the grains bounce away from strongly moving regions and collect along nodal lines.
In the late eighteenth century, Ernst Chladni sprinkled sand on metal plates and set them vibrating by drawing a violin bow along an edge. At resonant frequencies, the grains bounced away from strongly moving regions and collected along nearly motionless nodal lines. Modern demonstrations often use a speaker or electronic vibration driver so the frequency can be controlled precisely.