*This will only work for Americans unless you can transpose it to other countries.

Anyway, this one came off the top of my head when dealing with a co-worker who thought it was hilarious.

Basically you have to set the stage for her to notice your left-handedness (many women find left handedness interesting btw). I usually get ink on my hand "accidentally" and make a reference to it and being left handed.

Then tell her: "You know 80% of all US Presidents were left handed!? I can't guarantee that I will be president, but I will rule the world some day and because I like you so much, I'll spare your life. Heck, I may even make you co-ruler of the world."

Bam! You've demonstrated humor, story-telling ability, spontaneity, intelligence, and high value.