Have you ever been driving down the road and you flick a cig out of the window, then a few minutes later you smell something and turn around only to see your nan fingering herself in the back seat?
Did  you seriously see your nan doing that to herself?
Why should I be serious, it's only about nannys fingering themselves.
Jokes about your elderly relatives touching themselves are gross and based on their mental deficiencies could be true and hurtful. It would not be good if people make fun of you doing things without knowing when you are older.