Despite numerous comments to the contrary, it is possible to tickle yourself, but only via one method (at least that I am aware of):

Very gently rub your gum just behind your top front two teeth (on the sloping bit that leads to the roof of your mouth).

This leads to your body shivering in that " I have been desperate for a pee, and now it's out" way.

I feel sick.

I guess if you were really unlucky, you could get a screwworm, filarial worms and Guinea worms, whilst the "crabs" you suffer from thanks to that trip to the shop with no windows last year all get a sacculina carcini. Goodness only knows what they would do to your genitals!