Beneath your skin lay millions of tiny nerve endings that alert the brain to all manner of touch and exposure to things like heat and cold. It's this sense that allow us to keep from burning our hand off if we put it on a hot stove or to know we should put on a coat and another layer of clothing when it's freezing outside.
When these nerve endings are lightly stimulated -- for example, by another person's fingers or by a feather -- they send a message through your nervous system to your brain, which analyzes the message. The effect of a light touch that results in a tickling sensation is the result of the analysis of two regions of the brain. The somatosensory cortex is responsible for analyzing touch; for example, the pressure associated with it. The signal sent from the skin's sensory receptors also passes through the anterior cingulated cortex, which governs pleasant feelings [source: Blakemore]. Together, these two create the tickle sensation. This sensation seemingly results from a light touch: As anyone who's ever been tickled too hard can attest, too much pressure can cause tickling to go from pleasurable to painful.
We know these two regions are associated with tickling through the use of functional MRI (fMRI) studies. This technology also revealed why we can't tickle ourselves: The cerebellum, located at the back of the brain and responsible for governing movement, can predict a self-tickle and alerts the rest of the brain that it's coming. As a result, the intensity of the sensation is muted [source: Uhlig and Derbyshire].
Why would the brain do this? It may have something to do with sensory attenuation, the process by which the brain filters out unnecessary information in order to concentrate on the important stuff [source: Queen's University]. A predictable light touch from your own fingers appears to not be worth your mind's attention, so your brain discards the information before it has a chance to enter your consciousness.
The fact that you can't tickle yourself supports the idea that tickling is a product of socialization. Find out what the science of happiness has turned up about the social aspects of tickling on the next page.