A/N: I always thought that Slughorn was pretty oblivious. I wanted to show Snape from someone's viewpoint who didn't know about his love for Lily.
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Horace Slughorn walked slowly down the dungeon corridor, ducking briefly into his office to lay down a pile of scrolls that needed grading. He tread lightly for such a big man, though his step was intentional. He had heard someone enter his classroom as he came down the stairs, and no one but a student attempting to steal would do that on a Saturday.
He peered carefully around the door and nearly gasped in surprise. Severus Snape, the best student in his year, on par with young Lily Evans (though he certainly lacked her charm) was sitting on the floor, twirling a flask of something in his hand. In the poorly lit dungeon, the potion sparkled opalescently and the flask reflected the greenish light from the fires and the almost sourceless light that always seeped into the room somehow.
Severus was sitting in such a way that he could not see Slughorn, but Slughorn could see part of his face. He sniffed the potion briefly and a rush of emotions flitted across his face so quickly that Slughorn could not tell them apart.
The dungeon smelled better than it ought to—a bunch of third years had caused an explosion on Friday that had left a sulfurous, wet smell that had lasted for a few hours, then almost completely faded to leave a smell like rotting flowers. So why was there a faint aroma of candied pineapple and new leather and—it hit Slughorn in a flash. Severus had been brewing Amortentia, and what's more, he had, in his left hand, a strand of long, black hair. He twisted it nervously around his finger and suspended it over the flask of potion.
"Rrrgh!" Severus's face twisted as he half-choked a cry of pain and rage. He flung the flask against the wall. It shattered and fell with a musical jangling sound, the shards catching rainbows in midair. The potion left a dark stain on the grey stone floor, a couple pearly drops caught on the glass fragments. Severus stood up furiously and ground his booted foot into the broken glass until it turned to wet, sparkling dust; then he ran furiously out of the classroom. Slughorn wasn't noticed, but he noticed that Severus seemed to be crying tears of fury.
"Amortentia," he whispered softly. "Good Lord, what is that boy after?"
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"Sometimes I think, it's easy to brew, and if this is what it takes, but I don't want your love if it's not for real, oh I'm so tired of fakes" --"Amortentia" by Split Seven Ways.