So as not to offend the "delicate sensibilities" of some of my readers---and also so I don't freak my mom out; she still thinks I don't use THAT word--- I have changed the punchline of the joke below.
It really is hilarious though! Enjoy!!
"Dear Lord," the preacher began with arms extended and a rapturous look on his upturned face, "without you we are but dust..." He would have continued, but at that moment one very obedient little girl (who was listening carefully) leaned over to her mother and asked quite audibly in her high little girl voice, "Mommy, WHAT is butt dust?" ( or bottom sand or bottom ash or bottom soot, if those (terrible) replacements are more appealing to you.)