Why you can't tickle yourself

London, 1999. A robot gets ready to tickle sixteen volunteers.

Each held a handle in the left hand. Moving it made a second robot brush foam across their right palm.

So they were basically tickling themselves, by remote control. And it barely tickled, much less than when the robot moved the foam on its own.

The cerebellum, at the back of your brain, predicts what your own movements will feel like, and it uses that guess to turn the feeling down.

Then the team added a short delay between the hand and the foam. The longer it got, the more it tickled, and at a fifth of a second it matched the robot alone.

Your brain's prediction simply arrives too late, so your own hand gets treated like a stranger's.

So if you want to be tickled, you'll need a friend. Or a slightly slow robot.