All human laughter consists of variations on a basic form that consists of short, vowel-like notes repeated every 210 milliseconds. Laughter can be of the "ha-ha-ha" variety or the "ho-ho-ho" type but not a mixture of both. Humans have a "detector" that responds to laughter by triggering other neural circuits in the brain, which, in turn, generates more laughter. This explains why laughter is contagious.