Humour - Why can't I tickle myself?

This is a little off the beaten laughter path, but believe it or not, some research is being conducted in this area. In fact, researchers at the University of California in San Diego have even constructed a "tickle machine."
Some scientists believe that laughing caused by tickling is a built-in reflex. If this is true, then, theoretically, you should be able to tickle yourself. But you can't -- not even in the same area and the same way someone else tickles you into hysteria! The information sent to your spinal cord and brain should be exactly the same. But apparently, for tickling to work, the brain needs tension and surprise -- something that's obviously missing when you tickle yourself. How the brain uses this information about tension and surprise is still a mystery.