Because the brain of a person trying to tickle herself anticipates the sensations that will be caused by her fingers a process performed within the cerebellum the sensation no longer tickles.

You can, however, succeed in tickling yourself through an intermediary a machine, for instance, that translates the movements of your fingers into sensations on the skin by a method indirect enough for the cerebellum to be unable to anticipate them.

Over the years, the tabloid press has become increasingly intrusive, claiming the right not just to expose corruption and incompetence in high places, but to titillate readers with scandalous revelations about the private lives of the famous.