I think this is the most appropriate place to share this. I haven't figured out the answer yet, but I think I'm close. I couldn't find anything like this on the Internet, so I'm posting it here.
There are three travelers. They are inside a cave. Deep down in this cave, there are three doors, each door containing at least one diamond, and there are a total of 9 diamonds. Each traveler picks a door and gets the diamonds behind it.
The travelers loot their diamonds, and, before exiting the cave, all three must say simultaneously if they can deduce the number of diamonds that each of the other two have. They never lie, and all of them said they couldn't deduce it.
After these statements were said, one of the travelers realized that now he knows the answer.
So the question is:
How were the diamonds split between the three travelers? (the order doesn't matter)
I was going to write what I've managed to do so far, but I think it isn't a good idea to give away too much information even if its concealed by spoilers. This riddle was suggested by my discrete mathematics professor. It isn't homework and she didn't mention where it came from.