you were angry because of something in potion class when you said

You: "This is bullshit"

professor Snape heard you and gave you detention

Snape: "You can take that language straight to detention! Anyone else?"

the Slytherin boys we're minding their own business when mattheo intentionally knocked over his books and said

Mattheo: uh motherfucker!

Snape turned to him too

Snape: Fine detention to you too Mr Riddle!

mattheo turns to you and winks

You: just perfect you said sarcastically