One of the greatest and cruelest joys in the world is tickling. I mean, who doesn't enjoy making somebody laugh? Especially when they can't help but laugh. But when you think about it, tickling -- that's just a weird thing. What is that? Why does it exist?   The word "tickling" is actually an umbrella term for two different phenomena. The first we're going to talk about is knismesis. That's the kind of itchy feeling that you get when somebody puts a feather on your sensitive skin and it's like gnaaaah.    That sensation creates a desire to, like, rub or scratch, but it doesn't cause you to laugh. That kind of tickling is called gargalesis, which is caused by a harder touch or stroke that unleashes a confusing cocktail of pain and pleasure into the brain.    Now, knismesis makes sense. If an insect or spider crawls on your leg, you feel a tickling sensation, and you, you get it off. Knismesis is widespread in mammals: a horse whips its tail when a fly lands on its back, and a cat's ears twitch when you brush 'em.   Gargalesis, on the other hand -- and I love that word -- is only experienced by primates, which include humans. When you tickle a chimp, it lets out a noiseless pant that is the evolutionary precursor to a human laugh.   And all us primates are ticklish in the same spots: The ribs, under the arms, under the chin, near the femoral artery in the leg... these are all vulnerable areas of the body, leading some psychologists to theorize that gargalesis is an evolutionary mechanism that young apes and humans use to teach each other how to protect themselves if attacked, the equivalent of, like, puppies play-fighting.   We instinctively curl away from a tickle. We kick and twist, but we also laugh and smile, which is an encouraging facial expression which tells the tickler, "More! Please!" If tickling induced a negative facial expression, the thinking goes, then people wouldn't do it to each other, and we wouldn't learn how to defend ourselves.   So the next time you attack your brother with a storm of tickle-fingers, you can say, "Dude. I'm just teaching you life skills!"   Thanks for asking this fascinating question. Who knew it would be so interesting? And thanks especially to our Subbable subscribers, who get these answers a little early. If you'd like to get a sneak peek or other perks like a SciShow tie or a SciShow pocket protector, go to subbable.com/scishow. And if you've got a Quick Question, you can ask us on Facebook or Twitter or Tumblr or down in the comments below, and don't forget to go to youtube.com/scishow, and subscribe.