well you could start with the one I always use.
"Ummm.... well... I uh.... See there was this..... well... ummm."
If that doesn't work you can try:
the one I keep in my back pocket is sure fire, but you really gotta act it up some:
"Weelllllll duuuuuuuuuuuuuuh.. It's for the ALIENS. Hello!"
Then walk away muttering the question "why do I always have to explain myself?" just loud enough for her to hear.