Sirius when you talk back to a teacher.

You and Sirius were flirting across the classroom during one of professor McGonagall's lessons when she called you out.

"Miss (y/l/n), I would appreciate it if you and Mr Black could save your flirting for outside the classroom." She snapped at you, you looked away from Sirius to look at professor McGonagall.  

"Sorry, professor. I know it must be difficult for you to understand flirting since I'm guessing that you don't experience it your self." You answered, grinned. The entire class burst out into fits of laughter as professor McGonagall flushed.

As professor McGonagall shouted at you about detention, you looked over at Sirius who was laughing at winked at him.

"God, I really do love her." He thought as he grinned at you.