A Game With a Job

For young mammals, tickling works like sparring disguised as fun. The reflex only fires on unexpected touch, so a pup must stay alert and read who it can trust enough to play with. Tickled rat pups even run back for more, returning to the hand again and again.

That pull toward something you also squirm away from is the whole point: it keeps the game going long enough to rehearse a real skill.