Originally posted by THUDandBLUNDER
3) Why does a mirror appear to invert the left-right directions, but not up-down?
Because we tend to be standing up when looking at mirrors; we think of people as being able to turn freely about a vertical axis, but not around other axes; and we think of left and right as being somewhat similar, whereas up and down are different (because gravity pulls downwards), as are forwards and backwards (because we can only see forwards). When you look at yourself in the mirror, you're seeing an image of yourself reflected back-to-front, but this is too drastic a transformation for your subconscious brain to recognise it as the most obvious explanation for what has happened. Instead, your decide subconsciously to interpret what you are seeing as a copy of yourself which has been rotated 180 degrees about a vertical axis (because that's something people can easily do to themselves) and then flipped left-right (because who cares about left vs right?). Try lying on your side whilst looking at a mirror, and you'll find your subconscious miraculously stumbles across the correct interpretation of what you are seeing.
A spherically symmetric life-form would not understand your question, as it would correctly imagine that a mirror reflects things back-front (if you take 'forward' as 'towards the mirror', ie the direction in which the creature's attention is focused).