I posed this in Rinkchat yesterday and got called away before I could give the answer. Sorry.

Three guys, Al, Bob and Craig argue a lot about who is smartest. Their friend Dan, comes up with a test to see.

Al, Bob and Craig are sat around a table. They can all see each other, but since there are no mirrors or reflective surfaces they can't see themselves.

Dan goes around tapping each of them on the forehead. He explains that this may or may not leave an ink stain on their head. (They actually all have ink on their foreheads.)

Dan asks them all to start tapping on the table if they can see an ink stain on one of their companions and to stop tapping once they know if they have ink on them or not.

After a while Al stops tapping. How did he know he was inked?

A
N
S
W
E
R

B
E
L
O
W



The answer lies in Al trying to get a handle on what Bob is thinking.

Al's thoughts...

"Gee, I wonder if I have ink on my forehead? Hmm, Bob and Craig sure do. They're all tapping on the table, so I know they see ink on *somebody's* forehead, but they could just be tapping for each other. Oh, wait. If I didn't have ink on my forehead Bob would know that Craig was tapping for him and he'd have stopped by now. He's a smart guy, so it can't just be him not realising that.

Wow. I guess I have ink on my forehead."

SE