"Sneeze" on him
"Sir, sir! My potion is blue!" Snape turned to look and, sure enough, it was a bright blue. He walked over, and leant over their cauldron.
"Well, it comes to no surprise to me that the two of you are completely incompetent, as usua-" but he was interrupted as the red-head gave a loud sneeze. Conveniently, although Snape wasn't to know, this covered up his brother's muttered 'aguamenti'. Fred never did get the hang of non-verbal spells. Anyway, back to Snape.
The red-head with the bright blue potion gave a loud sneeze, and the front of Snape's robes were suddenly slightly damp.
"Oh, I am so sorry, sir!"
