Why do mirrors flip horizontally (but not vertically)?

Why do mirrors appear to flip images horizontally but not vertically? http://physicsgirl.org/ Instagram: http://instagram.com/thephysicsgirl Facebook: http://www.physicsgirl.org/facebook Twitter: http://www.physicsgirl.org/twitter Help us translate our videos! http://www.youtube.com/timedtext_cs_panel?c=UC7DdEm33SyaTDtWYGO2CwdA&tab=2 Featuring guest Meg Chetwood @megatronvolley Thanks to Kyle Kitzmiller and Veritasium http://youtube.com/veritasium Music: "Rhythm Changes," "Locally Sourced"