Or they walk up to you later and ask if you understood that math question that morning and you are like, "uhm.I wasn't here for that," and they are just like "really? I didn't even realize that you were gone." ^^So sad but so true
One time when I did this my pen flew across the room and smacked my friend in the face. I had the hardest time holding back my giggles while she stared at me so confused as to why I threw my pen at her.