Jellyfish sting when they're touched in water
Having grown up on the same beaches wow refers to, I always thought this was the case, and that Jellyfish only sting when in the water. In fact, many of the jellyfish on New England beaches are not poisonous, so we used to have some pretty amazing jellyfish fights as kids, during certain times of the summer when they were everywhere. I can attest that, in those circumstances, it was definitely a smack of jellyfish. I'd have to concur with Jazzo that the little girls were less interested in such activities than us nasty boys.
Our northern beaches get jellyfish ... much different from the Man-O-War type found further south!
It turns out that these beasties' stingers are pretty nasty for quite a while after they've washed up. I found a Man O' War about six feet across in Massachusetts when I was a kid, and my inexperience I didn't avoid the long tendrils coming off it. They looked like very long strands of red hair, but wherever they touched skin they hurt worse than just about anything - and taking them off didn't help a bit.
