"Hey! Hey, Remus! Look at this!" Potter screamed from the opposite end of the Potions Classroom. Lily's partner, Remus Lupin, looked up from his textbook to see what all the ruckus was about; she shot him a brief look of annoyance before stirring their potion- which he was supposed to be doing, mind you.
Lo and behold, James Potter was standing on a desk, one foot in his Cauldron and the other waving about. "If I take it out it turns yellow!"
He made a move to lift his foot when Sirius discreetly shot a tripping hex at his foolish friend. Potter slipped, landing on his arse in front of the Slytherins. Not 5 seconds later, the Cauldron followed the motion and landed on his inflated head.
Needless to say, Potter received a detention and became the laughingstock of the school for the next week.