A Tricky Question

Updated 05/02/06: See below.

Imagine you are standing in front of a mirror, facing it. Raise your left hand. Raise your right hand. Look at your reflection. When you raise your left hand your reflection raises what appears to be his right hand. But when you tilt your head up, your reflection does too, and does not appear to tilt his/her head down. Why is it that the mirror appears to reverse left and right, but not up and down?

This is allegedly one of the questions from actual interviews conducted by Microsoft. Some of the questions in that page are simply assessments of creativity and out-of-the-box thinking, and that means there are no "right" or "wrong" answers, but only "good" or "mediocre" answers. However, this question is different, and it really stumped me throughout. We all know that a mirror image inverses the left-right direction of an object, but why does it do so on the left-right axis selectively, without affecting the up-down and the forward-backward axis? This is not a "fancy" mind twister, it's in fact one of nature's peculiarity that stares at us all the time.

It took me several "day-dreaming sessions" to figure out a rigorous answer for that. All of a sudden I felt lucky for not pursuing careers that require tremendous ingenuity and creativity. Somehow I think that I have become blunter and slower in mental capacity as I grew up. Does the brain grow old? It might have been so in my case. Sometimes I even blame the medical course for "dumbing and numbing minds", but that was not exactly a good excuse.

Anyway due to time constraint I will just leave the post here and let any interested minds spend a few minutes in that question. I am interested to hear great and elegant answers from anyone. I will post my answer when I am free (if anyone is interested), but it might be a week or two from now, considering that I will fly back to Melbourne this Thursday and prepare for the new semester on next Monday. If you aren't interested in this particular question, the page I mentioned above contain many more questions that could keep you busy for weeks. So I invite everyone to spend some time twisting our brains to keep them sharp and shiny! :)

Update: I haven't had time to put up my own answer in my own words. However, when I was googling randomly I came across one of the explanations for the question. To be frank that explanation was too wordy to my liking so I didn't really finish it. But from what I got at a glance, it seemed like a great explanation. Have a look at some other Q&A inside too. Remember this question? "On Earth, from where can you walk one mile south, one mile east and one mile north and return to the starting point" The answer is surprising.