Why we can't tickle ourselves

As my roommate Dean used to joke, 'I can tickle you from the inside.'

Blatantly stolen from Fark.com and then found on CBC News.

Just before we make a movement, a signal is sent to the brain telling it what to expect. If the movement is anticipated, the brain filters it out, de-sensitizing us to the signal. That's why we can't tickle ourselves.