Why can't you tickle yourself?

You can't tickle yourself because your brain sees it coming. Whenever you move, your brain predicts what that movement will feel like. When you reach for your own ribs, the cerebellum, a part of the brain involved in coordinating movement, tells the brain's touch-processing area exactly where and how the touch will land. The sensation matches the prediction, and the brain turns the tickle down. Tickling seems to need surprise, and you can't surprise yourself.

That idea is ancient. Aristotle already asked why nobody can tickle themselves, and argued that tickling depends on surprise. Charles Darwin noticed that tickling only works if you don't know exactly where the touch will come next. In 1998, researchers led by Blakemore put it to the test with brain scans and a tickling robot: when people controlled the robot themselves with a joystick, they couldn't make themselves laugh.

There are a few loopholes. The light, crawling-insect kind of tickle doesn't rely on surprise, so you can give that one to yourself. And some people with schizophrenia can tickle themselves, possibly because their brains are less good at tracking the results of their own actions. For everyone else, the best tickler will always be someone else, even a machine: people laugh just as much when they think a machine is tickling them as when they think it's a person.