Molly, age 9, and Sammy, age 10, are sitting on the frontporch swing. Sammy says to Molly, "Screw you, Molly."
A minute goes by and Molly replies, "Screw YOU, Sammy."
A moment or two and Sammy says, "Screw YOU, Molly."
In response, "Screw *YOU*, Sammy," Molly says.
After about ten minutes of this, Molly's mom comes out onthe porch and says, "What on earth are you kids doing?"
They reply in unison, "We're having oral sex!"