The moment when the teacher just look at you as if you're doing something wrong.
The temptation of going up and tell the teacher to stop turns into laughing for no particular reason.
Teacher continue his/her stare.
That's when you realized you're doing what you're not meant to be doing.
(e.g. having your hair loose when near a bunsen burner, not wearing safety glasses near a bunsen burner, not having you notebooks out after the teacher instructed you to.)