...what would that be? Please listen, and don't laugh before you have finished reading all I have to say about it. On the one hand its embarrasing, but on the other, I can't keep that to myself any longer. Usually, I don't ask questions of such "triviality" to others, but instead prefer pondering on them until I figure them out - and then, I wouldn't be asking a question. But in this case, I think the problem is so weird (and maybe proof for my utter misconception of the world), that it deserves being asked before filed in the "problems I've solved" section of my brain. I'm on the best way to arrive at a final explanation, but until then: Enjoy (in case that you are just as stunned by this as me, I wont give you any hints what I've thought so far). So, what is it? The question is as trivial as the above introduction suggested. Maybe you see the solution at a first glance. I think you wont. However, don't make the mistake to put the problem just aside for being apparently trivial, unless you are really sure that you understand what it is. Why does a mirror swap Right and Left, but doesn't so with Up and Down? One may argue that the mirror doesn't actually swap left and right - since you see your left in the left side of the mirror, but consider the following: You see yourself in the mirror and you raise your left hand. Your counterpart raises his/hers right (in his/her reality). You move your head left while your counterpart moves it right. You move it up, however, and your counterpart complies. Still not convinced that there is an error in the universe? Imagine that (or just try it, your mirror wont lie to you): Stand in front of a mirror: Raise your left arm and describe a large circle arround your upper body. When your arm is left to yours, your counterpart has his/hers right to him/her. Same with right to you. The only point where you and your counterpart would agree on where your arms are, is when it's excactly up, down respectively. What is it that makes up perceive that? Gravity? The symmetrie of the human body? One may easily be tempted to say "yes" to one of the former. But that's not enough. Provide a thorough explanation or admit, that you don't know the answer to the absurd mirror problem.