A few months ago I was on the bus with DD2 (13) and it was one of those wet and miserable days when the windows were all steamed up.
A teenaged lad sitting opposite us wrote F**k of on the window with his finger and my DD was when I leaned over and said to him "I think you'll find there are two fs in off" .
The lad look a me a bit like this and then added the missing f .
I love being a pedant .