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