Q: 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?
A: This is because human is left-right symmetric and with well defined up-down. Imagine a red cross is put in front of a mirror floating in outer space, the reflection can be interpreted either as fixed left-right, reversed up-down, or fixed up-down and reversed left-right.
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