I never understand the proof by contradiction, because somewhere in the middle I always lost myself. In this https://www.physicsforums.com/showthread.php?t=523874 there's an example of proof by contradiction. We assume that if n^2 is odd than n is odd. This means that if n^2 is even, n can be odd or even. How can I proof a contradiction if n is even? It doesn't tell me nothing.