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Alice puts the coin on the table and covers it with a coffee cup
Bob chooses heads or tails
Alice lifts the coffee cup, revealing her choice
If Bob guessed right, he wins
It's fair even though Alice controlled the coin
Things can be fair even when they're not random
(Consider the I-cut-you-choose technique for dividing cake)
There are two important features here:
Alice has committed to her choice and can't change it
Bob can't see Alice's choice until after he makes his own guess
That's what the coffee cup is for
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Copyright © 2005 M. J. Dominus |