A Nancy Drew Mini-Mystery: Ned went to a party and drank some of the punch. Nancy wasn't there so he decided to leave early. Later, Ned found out that everyone at the party who drank the punch was in the hospital because the punch had been poisoned. Why was Ned not affected by the poisoned punch?
The solution is the poison from the punch came from the ice cubes. When Ned drank the punch, the ice was still frozen. Gradually, as the ice cubes melted the poison was released into the punch. Since Ned left early, he didn't drink the poisoned punch.
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This is a solid mystery, but I have to say that some of the comments were even more entertaining than the mystery. The most popular joke suggestion is that Ned poisoned the punch. The second most popular joke suggestion is that Ned has seen The Princess Bride, so he spent a few years building up an immunity to Iocane Powder.
Either way, it seems Ned should be more careful the next time he receives a party invitation.