PTO: please turn over

I feel terribly silly. My wife was helping Wolfie to do his homework last Sunday and, as there was no more room on the page he was writing on, she told him to write more on the other side. Shethen told him to write PTO, for "please turn over", so the teacher would know there was more written. And that's why I feel silly. Because I didn't know. I mean, I heard it, I had seen it, but I never pay attention to what PTO meant in that context. So yeah, anyway, PTO is the word (abbreviation?) of the day.