What is the game of duck-on-a-rock?

Duck on the rock is a pub game in which a glass is placed on a table, and a player must balance a coin (sometimes called a "duck") on the rim. The coin may be flicked or dropped onto the glass. If the coin stays balanced, the player repeats with an additional coin; if the coin(s) fall off the glass, the player must drink a specified amount of alcohol and starts again with one coin.