What is a good but safe prank to play on a teacher?

What is a good but safe prank to play on a teacher?
This is a really funny but simple prank I did during Maths once and people said I was funny for doing it.
1. When the teacher's not looking and the classroom is SILENT, cover your mouth with your hands and say something really random. I said one of my friends' names. Say something like "Daddy" in a kiddy voice or a strange catchphrase! ('Uh oh, spaghetti-o!' in an Italian voice might be good!)
2. Do this two or three times a day during different lessons, continue it over the course of the next 30 days but keep them for really random moments.
3. Don't be afraid if someone "copies" you! The more people involved the better and funnier it can be!