disk containing a shallow layer of fine sand and covered with
rigid transparent plastic is knocked with a rubber mallet. Strange
patterns appear when the fine sand is vibrated. When the observer
tilts the disk and continues knocking, the sand will slide down
like a miniature avalanche. Knocking the disk causes the sand
particles to vibrate. The vibration reduces the friction between
the sand particles and allows the sand to flow like liquid. The
process is similar to liquefaction of sediment during an earthquake.
The patterns that appear are the result of complex interactions
between the flowing sand and the vibrating air inside, set in
motion by the moving walls of the container.