In the first Potions lesson Snape put it down to chance. There was a large part of him that refused to accept the possibility that Harry Potter might actually be good at Potions. That he might be his mother's son, not just his father's. Snape prided himself on his ability to use cool logic and to not let mindless emotions interfere, but in the case of Lily Evans and James Potter, he had always failed.
In the second lesson he found it harder to convince himself. Potter seemed to possess that curiously random gift that Lily had had. He hardly seemed to pay attention to his cauldron, and yet the potion ended up perfect.
In the third lesson he watched Potter like a hawk and noticed something quite unusual. When Potter obviously felt that he wasn't looking, Potter was whispering hints and tips to his friends. Snape chided himself for not noticing it sooner; everybody in Potter's immediate vicinity was producing passable potions, whereas normally most idiotic First Years in their first few lessons would be lucky if they managed not to melt their cauldron.
It was in the fourth lesson, however, that Snape's curiosity really began to get peaked. Potter's potion was decidedly and absolutely average. Snape had been watching as closely as he had the previous day and he knew that Potter could have made his potion better than he had. Potter had deliberately sabotaged his own potion, not so much that it would receive one of his sneering remarks, but, if he hadn't already been paying special attention, it would have fallen just below notice.
The very fact presented Snape with an enigma. He would have to accept the fact that the spawn of James Potter did not crave the attention he received, in fact, he actively sought to avoid it. In fact, the action was unlike both of his parents, for even though Lily was not an attention whore like Potter, she enjoyed receiving praise for her work as much as any person. Potter was an enigma.
Snape had always had a fondness for logic puzzles.
Snape continued to watch. Potter continued to be frustratingly average. Snape could not figure it out, and so after one lesson he told Potter to stay behind. "There is a most curious thing I have happened to notice, Potter." Potter stayed silent, "You are sabotaging your own potions,"
"You always give me decent marks," Potter replied, a slightly alarming, panicked look coming into his eyes.
"I should be giving you extraordinary marks. I saw your potions at the beginning of the year. Tell me what is going on." It was said with such an underlying note of threat that, after a long moment's panic, Harry knew he had no other option but to tell the truth.
"I … I don't like to stand out."
Snape held Potter's gaze for a long time. He hadn't noticed earlier, but now he looked, the signs were all there. Potter was small … underfed. He tried to keep below anybody's notice and seemed perpetually on guard. At first Snape had attributed Potter's constant alertness and vigilance to a desire to fight and show off, much like his father. In fact the truth was the exact opposite. Snape resisted the urge to sigh. This was not his problem. And yet it was, not least because this was Lily's son but that he, like so many others would and had, judged the boy on the image rather than the reality. Besides, he had already gotten himself involved when he decided to investigate.
"Potter," he said brusquely, "you are, in a word, a genius at potions. You possess an instinctive grasp and an ability to channel your magic into the potion that very few possess and I will not let you put that to waste. I expect you to work to your full ability from this lesson forward, do you understand me?" Harry nodded wordlessly. Snape sighed, Here goes nothing, "Potter," he said, "I understand the desire to just hide away, but by doing that you let them win," and Harry, like the good little predictable Gryffindor he was, gave a determined look and nodded again,
"I understand, sir,"
Snape let him out of the dungeon and watched him leave. This, he thought, is going to be an interesting seven years. Good, I was getting bored.