If someone makes a grab for that ticklish place above your knee, you double over in laughing protest. When you grab your own knee, nothing happens.
You can’t tickle yourself because your brain doesn’t pay as much attention to sensations caused by your own actions. In fact, brain scans show that neurons in the cerebral cortex—the part of the brain responsible for attention, awareness and consciousness—are less active during a self-tickle than when someone else is doing the tickling!
Scientists say the brain distinguishes between expected sensations caused by our own actions (for example, the pressure of the keyboard against our fingertips when we type) and unexpected sensations from our environment (for example, if someone sneaks up from behind and taps us on the shoulder). The reason? Heightened awareness of unexpected contact from our environment helps keep us safe.