Stretch a rubber band between your two thumbs and then pluck it with your index finger. Better yet, stretch the rubber band in front of a windy fan to get it vibrating. Note that the area of greatest...

Stretch a rubber band between your two thumbs and then pluck it with your index finger. Better yet, stretch the rubber band in front of a windy fan to get it vibrating. Note that the area of greatest oscillation is always at the midpoint. This is a self-reinforcing wave that occurs as overlapping waves bounce back and forth from thumb to thumb.