Yesterday a collegue in office gave me something to think about in order to re-activate my brain after my lecture. Here it is.

Ten balls are ligned up in front of us and we play the following game. I mark mentally two neighboring balls and you must find out which ones. To do so you have the right to name two subsets of the balls and I will tell you how many of my marked balls each one contains. The question is: Can you win? If yes how?

I found the answer but it took me some time. Probably you will not do it in ten minutes ;-)