The bell rang.
The students began packing up to leave as though they had been granted a pardon.
"Clean your workstations! I do not want to see any residue left behind!" Snape said sharply.
He walked over to Douglas, his eyes scrutinizing him.
"Professor Holmes, I hope sitting in today has made up for your..."
"I learned a great deal, Professor."
Douglas stood up, cutting him off.
"Your laboratory renovation was very successful. Precise, efficient, and full of artistry."
His praise was sincere.
The corners of Snape's mouth lifted slightly, but before he could continue,
Douglas changed tack.
"However, your ventilation system doesn't seem to handle the smell of dragon blood very well. I'd suggest adding a Directional Adsorption Charm to the rune array. Oh, and the temperature-control runes on the obsidian workstations have a response lag of 0.5 seconds. For advanced potions, that could be fatal..."
Like a superior conducting an inspection, he pointed out several minor but genuine problems.
Snape's face twitched.
"No need to concern yourself, Holmes!"
"We're learning from each other."
Douglas brushed off his robes and prepared to leave.
As he passed the lectern, his steps paused briefly.
His body happened to block Snape's view.
His hand swept across the lectern so quickly that the movement was almost impossible to see.
"Goodbye, Professor."
Smiling, Douglas walked out of the classroom.
The iron door slid shut behind him.
Snape stood in the empty classroom.
He kept feeling that something was wrong.
He began inspecting the classroom.
It was his habit.
He had to ensure that every ingredient had been properly stored and every cauldron washed clean.
He checked the precious ingredients with particular care.
Salamander blood, powdered Bicorn horn, mandrake roots and rhizomes...
Nothing was missing.
Snape breathed a sigh of relief. The man had once