Back in high school my biology teacher explained it like this.

Your brain is wired to ignore the totally expected, like tickling or pinching yourself or getting yourself wet.It's because the brain needs to be able to ignore predictable everyday sensations be ready for more important sensations, like bugs crawling on you. Life would be very different (and difficult) if you went nuts every time the soles of your feet touched the ground or your shirt brushed back and forth on your body. Instead, your brain causes you to react only to unexpected sensations like a spider crawling down your neck.

Why can someone else tickle you then?For some reason the brain can’t predict a sensation caused by an external factor, like someone else tickling you, even if you know it’s coming. That means if someone else tickles you, there’s no warning message sent to the brain and you end up feeling tickled!

That's all fine and well. Now can someone explain how my friend Manny can be tickled without any actual "tickling" taking place? Is his neurochemistry "jumping the gun"?