As a part of the bet you've had with your best friend from the Snakes' House, Regulus, you sneak into the dungeons and then into Snaps' office, slipping a batch of Euphoria Potion into wine that Snaps is particularly fond of. Then you go for your potions class, knowing that Snaps likes to have a little wine before. Since you cleverly neutralised the flavour so the Euphoria is undetectable even for the master of potions, Snaps drinks a little and soon he becomes bubbly and giggly, so unlike himself, basically prancing into the classroom like a merry child on Christmas Eve. Of course, everyone present is staring at him in shock, as his nose tweaks and he laughs uncontrollably, beginning to sing Christmas carols.