Why does a mirror flips only horizontally? why should there be a
"preferred" direction for flipping?

Hello.

Here are some articles on this exact subject:

"Why does a mirror exchange left and right but not up and down?"
Howstuffworks.com
http://science.howstuffworks.com/question415.htm

"Why do Mirrors Reverse Left and Right?"
http://math.ucr.edu/home/baez/physics/General/mirrors.html

"Why don't mirrors reverse up and down?"
http://at.yorku.ca/cgi-bin/bbqa?forum=ask_a_topologist_2001&task=show_msg&msg=0129.0002

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search strategy:
mirror "left and right" "up and down"

I hope this helps.

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Clarification of Answer by juggler-ga on 05 Jul 2004 15:21 PDT

My friend and colleague Pinkfreud-ga recommends this book if you'd
like more information:

The New Ambidextrous Universe: Symmetry and Asymmetry, from Mirror
Reflections to Superstrings
by Martin Gardner 
http://www.amazon.com/exec/obidos/tg/detail/-/0716720930