Why you can't tickle yourself (and how a robot can fix that)

Every time you move, your brain keeps a copy of the command and uses it to predict what you're about to feel. Sensations that match the prediction get turned down. That's why your own fingers can't tickle you: the brain saw them coming.

In a clever 1999 experiment, people tickled their own palm through a robot arm. When scientists delayed the robot by a fifth of a second, or twisted its direction, the touch became more ticklish, almost like someone else's. The same trick explains why tit-for-tat shoves escalate, and why some people with schizophrenia can tickle themselves.