Why are we ticklish? Rats are surprisingly ticklish when their mood is right

Of all physical sensations, ticklishness is perhaps the most mysterious. Why do we laugh in response to tickling? Why are certain body parts more ticklish? Why can we not tickle ourselves? Indeed, the mystery of ticklish perception has been discussed for more than two millennia by great intellectuals including Aristotle and Charles Darwin. Despite such long-standing interest, the mechanism of ticklishness remained elusive.