One way that is guaranteed to cheer up a tired, cross child or to reduce an adult to a simpering fit of giggles is a few light strokes on the sole of the foot or the armpit.
Some people are more tickly than others, and ticklishness varies from situation to situation.
Go on - try it. The sensation produced will never rival that of an external stimulus. Studies using functional magnetic resonance imaging, a technique which shows changes in brain activity in different situations according to cerebral blood flow, have shown increased S1 activity in response to an externally produced tactile stimulus. However, the same activity is not produced after an identical, self produced stimulus. We need to turn our attention away from the cortical areas of the brain to the cerebellum, a structure attached to its base, to understand why this is the case.