One thing we know about tickling: It makes us laugh. But why? And what exactly is it? Is it pain? Pleasure? Aristotle wrote about tickling all the way back in 350 B.C. Darwin did too, in 1872. But, of course, tickling’s been around for as long as big brothers and sisters have needed a way to torture their younger siblings. We get a primer on the strange, ancient science of tickling.