No conclusive evidence exists to support whether everyone is truly ticklish or not. Each person has a "ticklish" response to stimuli dependent upon the sensitivity of his nervous system and other factors, but tickling the skin does not always result in laughter.
Tickling is subjective, with stimuli producing a variety of responses. Unanticipated tickling often results in laughter, while being prepared for tickling can prevent the laugh response.
Gargalesis is tickling that can produce laughter, while knismesis is tickling produced by light touch that often results in an itchy feeling.
Various studies on tickling have been carried out, with a wide array of results that explain why tickling happens, how different people respond and why self-tickling does not work.