Bloody knuckles is a game in which two players use a coin (usually a quarter) that is spun around a flat surface (usually a table). The players continue to keep it spinning by flicking it with their fingers. The other player must touch it without causing it to stop spinning and fall on the heads or tails side. Then the person who spun the coin first tries to touch it next. This turn taking continues until the coin falls. Whichever player's touch causes the spinning coin to stop and fall down is considered the loser, and they must put their knuckles on the playing surface. Then, the winner puts their thumb on the coin, and forcefully pushes it across the table onto the loser's exposed knuckles. The game is played until someone's knuckles are bleeding or suddenly they quit due to excessive pain.
There are different variations of the game. One variation is where each player make a fist with the thumb wrapped around the other fingers. Then each fist punches the other fists. Players who flinch are out of the game. Whoever lasts the longest before quitting wins the game.