Riddle of the Week

Riddle of the Week: The Spies With Stamps on Their Heads

Difficulty level: Hard

By Jay Bennett

This puzzle is similar to Riddle of the Week #5: A Life-or-Death Question About Hats, but it is slightly harder and requires another layer of deduction. If you haven't solved the Hats Riddle yet, or if you are having trouble with this one, it might be worth revisiting the prisoners with their black and white headwear to get on the right track.

Now on to the riddle.

PROBLEM

Three super spies are caught sending sensitive information to an enemy state. These three double agents are apprehended and taken out to a remote spot in the woods. They are told that one of them will be part of a prisoner exchange, and the other two will be executed.

To decide who lives, the guards decide to play a game. They show the captives eight stamps: four red, and four green. They then blindfold the three men and stick two stamps to each of their foreheads. One of the guards puts the remaining two stamps in his pocket.

The guards then take the blindfolds off the captives, who can each see the stamps on the other two men's heads, but not the two stamps on their own head, and not the two stamps in the guard's pocket. These spies are highly intelligent—they're perfect logicians who know they can count on each other to correctly and quickly interpret the information they have.

The guard captain tells them that the first man to figure out the color of the stamps on his own head will be used for the prisoner exchange, and the other two will be executed. If anyone guesses wrong, they will be shot dead on the spot.

The captain then asks the spies in order if they know what color stamps they have on their head. The answers are as follows:

• A: "No."
• B: "No."
• C: "No."
• A: "No."
• B: "Yes."

Spy B answers correctly. What color are the stamps on his head, and how does he know?

HINT

Don't forget about the stamps in the guard's pocket.

Check back Fri to find out the answer