Tickling

Tickling is the act of touching a part of a body in a way that causes involuntary twitching movements or laughter. The word  "tickle"  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.

The question as to why a person could not tickle themselves 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 disagreed. When tickled, noted Bacon, "men even in a grieved state of mind, yet cannot sometimes forbear laughing."

When considering tickling in terms of its qualities as a sensation, it results from a mild stimulation moving across the skin. The dual character of tickling refers to the associated behaviors that include smiling, laughter, twitching, withdrawal and goose bumps. The tickle can be divided into two separate categories of sensation, knismesis and gargalesis. Knismesis, also known as a "moving itch", is a mildly annoying sensation caused by a light movement on the skin, such as from a crawling insect. This may explain why it has evolved in many animals. Gargalesis reactions refers to a pleasurable, laughter-provoking feeling caused by a harsher, deeper pressure, stroked across the skin in various regions of the body. These reactions are thought to be limited to humans and other primates, although some research has indicated that rats can also be tickled in this way.

...
Wikipedia