I too, once wondered this question & have since then, done some research on it. As surprising as this may come to you, the reason you laugh/giggle when tickled is because you & your body are in a state of panic. When somebody tickles you & you laugh & squirm, although you may not realise it, your body is actually in a state of panic & doesn't know what to do. The reason you squirm is because your body is trying to escape from whatever is tickling it, not knowing whether or not it's going to come to any harm. As for the laughing/ giggling, you may not know this, but your nerves that feel the sensation of tickling are also your nerves that feel the sensation of pain (this next part may be a little confusing to explain, so here goes). Since soft touches don't hurt you but they still tickle, your body & nerves become confused & can't tell whether or not you are in any real danger, even if you consciously know you're only in a playful tickle- fight, for example. Therefore, your body doesn't know whether to laugh or scream, so you kinda end up doing both, depending on how ticklish you are. As for me, I don't usually start screaming unless the tickling becomes very unbarable, which is rare for me as I LOVE BEING TICKLED, LOL! I hope I've helped you. I'm confused myself now, LOL!