Veer-Zaara

Zaara, age 9, and Veer, age 10, are sitting on the front porch swing. Veer says to Zaara, "Screw you, Zaara."
A minute goes by and Zaara replies, "Screw YOU, Veer."
A moment or two and Veer says, "Screw YOU, Zaara."
In response, "Screw *YOU*, Veer," Zaara says.
After about ten minutes of this, Zaara'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!"