There is a ball in a box and three kids are guessing what colour it is.  Val says: "The ball is~red."  John says: "The ball is~green."  Elvis says: "I agree with~John."  They open the box and find only one of them guessed right.  What colour is the ball? 
Overseas Competition->Knowledge Point->Combinatorics->Logical Reasoning->Reasoning by Comparing
We can spot that John\textquotesingle s guess and Elvis\textquotesingle{} guess are the same, so both of them must be wrong. Therefore, Val guessed it right. 