You stand before a full length mirror. Your image appears reversed horizontally, left for right, but not vertically, top for bottom.
Everything is directly opposite its image. Left hand faces left hand, head faces head, etc.
Your left hand faces the right hand of your image, but is is really your left hand. Paint a fingernail or do something else to distinguish it. Left faces left.
It seems funny because we are more or less left-symmetric, not up-down symmetric. If you lie on one side, the "reversal" is now top-bottom. Your left hand is up and your image's right hand is up, although it is still your left hand, as the painted fingernail reveals.
I just looked in the mirror and nothing was staring back. It's getting late...
