Knismesis and Gargalesis

There are two separate tickles. Knismesis is the light, creepy-crawly itch that makes you swat at your arm — likely an alarm for insects on your skin, which is why you can trigger it yourself. Gargalesis is the heavy, laughing kind, and it needs another person.

Gargalesis fails on yourself because your brain predicts your own hand far too well, leaving no surprise to react to. So one word, tickle, hides two different systems: a private bug detector and a heavier reflex that fires only when the touch is unpredictable — something that is hard to manage on your own hand.