Forget complex math problems, logic puzzles, memorization. The hardest thing you can try to do with your brain is to not think about something. It's virtually impossible. But why? As New Scientist explains, it has to do with what thoughts are actually made out of.
Your brain is a ceaseless hum of electric static as all your neurons shout back and forth to each other. But when you think—really think—all those crackles and pops coalesce into something more definite and specific. Beyond that, it's hard to tell exactly what's going on, but the thought to not think about "x" necessarily makes that thought physically appear in your brain. Tough luck.
It's a little trippy when you get into the nitty gritty of what your brain really is, isn't it? Best just to try not to think about it. [New Scientist]