Bogies.
Harry knocked on the door of the Potion Master's office for his very unnecessary detention. He didn't intend on Ron's hair staying luminous green, it was a mere mishap. Harry's potion had gone so horribly wrong; Ron actually had to shave his hair until it was thin enough to be able to be fixed!
"Enter." Severus Snape murmured tonelessly. This was a rather pointless detention, it were by accident his sidekicks friend had green hair.
"I'm here for my detention sir." Harry replied coldly. Hopefully Snape would get bored of torturing him soon and let him go. Harry shook his head stupidly. This was Snape he was talking about.
"Yes Potter – there is no need to state the obvious, you're wasting my time as it is. You will be writing lines for me. If you complete them before nine, which I daresay would be a miracle, you can start on cleaning these filthy cauldrons. You shall write 'I will not disturb my Potion Lessons for they are highly valuable. 50 times.'
"Yes Sir." Harry consoled himself. 50 times was reasonable.
The young Gryffindor sat on the front desk seat and began his lines. By the 39th line, his hand was beginning to ache, and his handwriting was appalling. It was only seven thirty. Willing himself further, Harry continued. By eight fifteen, Harry had completed the dreaded lines. Relieved, he looked up to show Professor Snape. However, Professor Snape, was not there! Snape was leaning forward in his filing cupboard both doors covering what he was doing. Quietly, Harry got up from his chair, without scraping the floor and tiptoed towards him. Upon further inspection, Harry could not believe his eyes. The Potions Master of Hogwarts had his fingers up his nose, and was picking at it one finger at a time!
Snape must have gathered someone else's presence, for his head whipped round and saw Harry shocked still in front of him.
"Potter!"
"Sir..." Harry choked out.
"You will not speak of this. Ever. Do I make myself clear?"
"Yes." Harry cleared his throat. "Uh. Why would you…"
"Well something has to get rid of those insufferable bogies!"