Tickling is the act of touching a part of the body so as to cause involuntary twitching movements and/or laughter. The word "tickle" (help·info) evolved from the Middle English tikelen, perhaps frequentative of ticken, to touch lightly. The idiom tickled pink means to be pleased or delighted.
In 1897, psychologists G. Stanley Hall and Arthur Allin described a "tickle" as two different types of phenomena. One type is caused by very light movement across the skin. This type of tickle, called a knismesis, generally does not produce laughter and is sometimes accompanied by an itching sensation. Another type of tickle is the laughter-inducing "heavy" tickle, produced by repeatedly applying pressure to "ticklish" areas, and is known as gargalesis. Such sensations can be pleasurable or exciting, but are sometimes considered highly unpleasant, particularly in the case of relentless heavy tickling.
The question as to why a person could not tickle him/herself was raised by the Greek philosopher Aristotle.
Francis Bacon and Charles Darwin believed that humorous laughter requires a "light" frame of mind. But they differed on ticklish laughter: Darwin thought that the same light state of mind was required, whereas Bacon said no: When tickled, noted Bacon, "men even in a grieved state of mind, yet cannot sometimes forbear laughing."