Three condemned prisoners share a cell. A guard arrives and tells them that one has been pardoned.

“Which is it?” they ask.

“I can’t tell you that,” says the guard. “I can’t tell a prisoner his own fate.”

Prisoner A takes the guard aside. “Look,” he says. “Of the three of us, only one has been pardoned. That means that one of my cellmates is still sure to die. Give me his name. That way you’re not telling me my own fate, and you’re not identifying the pardoned man.”

The guard thinks about this and says, “Prisoner B is sure to die.”

Prisoner A rejoices that his own chance of survival has improved from 1/3 to 1/2. But how is this possible? The guard has given him no new information. Has he?

(In Mathematical Ideas in Biology [1968], J. Maynard Smith writes, “This should be called the Serbelloni problem since it nearly wrecked a conference on theoretical biology at the villa Serbelloni in the summer of 1966.”)