Here goes another brainteaser:
You have an empty square of 3x3. Just like a sub-square of a sudoku. Inside, in eace one of the 9 boxes, you can only place one number. What numbers? 1, 2, 3, ... to 9. You can only use them one time. In fact, the entire square must be full at the end of the game (9 boxes for 9 different numbers).
The objective: sum of the numbers in ALL directions (diagonal, straight, lateral... but always lineally) must be equal to 15.