This is a really simple mathematical game, but it can get interesting once you add some variations. Players count to 21, they can say either one or two numbers in a row, but they can't say "10-11" together (because that adds up to 21). Whoever says "21" consumes. They then begin the next round.

E.g. "1", "2-3","4","5-6","7-8"... so on. Once players have mastered that, take them to Decrementing Total 21. The same rules as above, but after the first round the game becomes "20", i.e. whoever says 20 drinks. Whoever says 10 also drinks, because 10 + 10 = 20. Then the game becomes 19, and whoever says 9-10 drinks. This continues until some poor bastard says "1" (he will get the 2 and 3 rounds as well; think about it).

The next variation is to change direction every round.

The ultimate version of this game I've heard of is some Otago guys I met who played "All Black Team 1987" and, instead of saying the numbers, had to say the players (e.g. "McDowall","Fitzpatrick", etc.). They did it with different years and teams (obviously including reserves to get the 21).