Thursday, November 11, 2010 1:53:55 PM
Reminds me of a trick I learned the other day from a professional logician who probably would rather not be named here.
You offer your victim a $10 bill and a $20 bill, under the condition that if the next thing you say after they accept is true, they have to give back the $20. Either way, they keep the $10, so they have an incentive to go along.
Once the (female) victim agrees, you tell her "You will give back the $20 if and only if you take your shirt off."
For stumped non-logicians, "if and only if" works out true when both sides are true (or false), and false when only one side is true. So in both cases, the rules require the victim to remove her shirt.