Thimblerig

"You know, Thimblerig? The shell game? First you put a pea under one of three nutshells… And then there's a bit of sleight of hand as you quickly shuffle the shells… And then whoever's playing guesses where the pea's at… And then you lift up the shell they guessed… And then uh-oh, no pea? You shuffle the shells again… And then whoever's playing guesses where the pea's at… And then you lift up the shell they guessed… And then uh-oh, no pea again? You shuffle the shells again, and then…"

- Murphy McNully teaches the Thimblerig Shuffle to Jacob's sibling

Thimblerig is a game (or, more often than not, a confidence trick) in which three identical containers (such as shells) are placed face-down on a surface, a small ball (such as a pea) is placed beneath one of these containers so that it cannot be seen, and they are then shuffled by the operator in plain view so that a player is invited to bet on which container holds the ball.

Murphy McNully drew inspiration from Thimblerig when devising a Quidditch tactic of his own, which he called the Thimblerig Shuffle.