Scientists solve mystery of why being tickled feels better if someone else is doing the tickling.
The reason is that one part of the brain tells another what is about to happen and gives the game away.
Researchers at the Institute of Neurology in London made the discovery by seeing how tickling affected the brains of three volunteers.
First the subjects tickled themselves on the palm of the hand with a special `tickling stick' device, likened to the one popularised by comedian Ken Dodd.
Then a researcher operated the tickler. Brain activity was monitored using a magnetic resonance imaging (MRI) scan.
All the volunteers rated the self-administered strokes as less ticklish than externally administered ones - and the MRI scans showed why.
When someone else did the tickling there was much more activity in the somatosensory cortex, the part of the brain that processes tactile sensations. Another area linked to pleasure sensations lit up when tickling was received, but not when it was self-administered.
However only self-tickling activated the cerebellum, a brain region involved in planning.
The team suspected that when people try to tickle themselves, the cerebellum ruins the fun by sending urgent messages to the somatosensory cortex warning that a sensation is on its way.
To test the theory, they re-timed the tickling machine so that self-administered tickles came a fraction of a second later than the volunteers expected. This resulted in a self-tickle which was just as good as an external one.
Ms Sarah-Jayne Blakemore, who led the experiment, told New Scientist magazine: "The ticklishness increased as the delays increased." She now hopes to test volunteers with schizophrenia.
It is thought schizophrenics will be able to tickle themselves even without the delay, since their brains do not predict sensory stimuli in the normal way.