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Buffalo
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The rules are simple: 
Everybody must use their left hand while drinking
(except if you are left handed). 
If you see anyone drinking with the right hand, 
you begin shouting BUFFALO! BUFFALO! BUFFALO! and 
clap your hands or bang a table in the same rythm. 
Now everybody notice and join in. 
This means that the unfortunate drinker must finish 
his/her drink as fast as he/she can to make you stop. 

There are no exceptions from finishing the drink. 

This game is wery popular in Iceland (the home of drinking).