"A Tea Bag?"
"It's when a girl sucks on a guy's balls!"
"Oh. They have a name for that now. How nice." Then, as an after thought, "It doesn't have to be a girl."
Naturally, after that calmed down, the rest of the class wanted to know what a Sixty-Nine was. So I told them to write the number, on its side. The circles are heads, I added. It was apparently that kind of day. Rest assured, your children are learning with me.