You and Merula were working on a potion together, arguing and judging each other's techniques as usual.

Yet, this time Merula seemed to be rather tense than cocky, and the little arguments heated up really fast. Before you knew, Merula had broken one of the vials right on the cauldron, causing it to explode. No one was harmed...but you two were now covered in black smoke.

Both of you headed to the bathroom -after a looong 'talk' with Snape.- Merula frowning angrily, not even looking at you.