The recent discovery that rats can be tickled suggests a deeper understanding of human laughter. We laugh when a system we share with other mammals is disinhibited. Laughter can thus be induced by direct action on the brain. In ordinary life, we have one physical response, laughing, but it can be induced by three very different stimuli or circumstances. One, we laugh when tickled. Two, we laugh for purely convivial, social reasons. Three, we laugh at jokes, wit, and less intellectual stimuli-like pratfalls and practical jokes. Leaving direct action on the brain aside, we can hypothesize that the latter three all involve the same psychological situation. Laughter is induced by, first, a mild, sudden, and playful threat to our ongoing process of maintaining and re-creating a personal style of responding to many things besides jokes followed by, second, the nullification of that mild threat.