The first time Harry Potter had realized, it had been on that critical night of his sorting ceremony.
He just got into the ritual of eating and half-listening to Ron and Hermione's words and decided to examine the people he would have to spend several years with. You could tell a lot about a person by how they eat.
For example, Vernon ate like a pig getting food all over the place including his face. This showed that he not only had no manners but he didn't care about the taste of food or what he looked like for his family. Duddly ate similarly. Petunia ate in small bites always glancing out the window making sure the neighbors weren't looking. This showed she only cared what other people think about her and her family.
The Slytherin's ate with a sort of ingrained manner. They were quiet, not bothering with useless chatter. They used all their utensils properly and politely. They didn't grab any more then they could eat and didn't waste a crumb. This obviously signified they grew up with parents who cared how their children looked in the eyes of the public and that they knew the value of the simplest needs like food and shelter.
The Ravenclaw's were also polite and had good manners. Most of the firsties listened with rapt attention as the older years pointed out teachers and what they taught and how they taught. While the older years (well, those who weren't explaining, anyways) were whispering in heated debates about one subject or another. This signified that although they were polite they weren't afraid of letting people know their opinion.
The Hufflepuff's were also a quiet sort, like the Slytherin's, but they were quite for another reason. You could tell they didn't want to be like the Ravenclaw's arguing with everyone. They ate civilly. They looked like they didn't want to offend the people who cooked the food for they too were careful in what they grabbed and didn't waste any of it.
Now the Gryffindor's, he was ashamed to admit, were the loudest of the group. They had to have made up at least half of the noise in the Great Hall, if not more. They talked with their mouths full, ignoring the simplest of etiquette, like say eating with the utensils! They were rowdy arguing loudly about this or that not even thinking to notice if they were bothering anyone else. This showed they grew up with parents who either didn't mind a blatant disregard for the rules or who let their children go too long without order.
After scanning the student population he moved his eyes to the head table hosting all the teachers that would be stuffing his brain with knowledge for the next seven years. Most of the teachers were chatting politely with each other, enjoying their meals, enquiring about their summers, and asking whether or not they got the latest edition of some book or another.
Hagrid was speaking enthusiastically with professors Dumbledore and McGonagal, who were listening with slightly less enthusiasm. Professor Quirrel, who he had met in the three broomsticks, was talking to a sullen professor with greasy black hair that tried his worst to look interested in the subject at hand.
It was then, in that half-a-moment, when their eyes locked, that he realized that they were the same. His eyes started to widen a fraction but thankfully in that second half-a-moment a searing pain ripped through his scar burning his eyes making them water before his professor could come to the same conclusion he had. And for that he was eternally thankful.
He quickly turned away jamming his eyes shut and hissing in pain. He held his hand to his forehead as he waved off Hermione and Ron with a forced smile before getting back to his meal. He wondered if that professor saw how similarly he ate to the Slytherin's…But this only drew him back to what he wished he hadn't saw…
His future professor's eyes held the same trapped, hopeless look his eyes held when he looked in the mirror.
Perhaps he should change the way he ate, after all how a person ate did tell a lot about them…
The only time Severus Snape even came anywhere even close to realizing it was when he was teaching his first potions class.
He made sure to fill his voice with contempt for the boy he hated, the boy that should have been his. He glared at the boy who was sure to spend so much time getting whatever he wanted that he wouldn't have thought to study for his classes. But just to be sure he asked him seventh year questions; really he knew these answers when he was in first year, why couldn't this brat study for once in his miserable life?
The boy floundered around at a loss for words making him smirk mentally. The brat really didn't know anything. Then he had to be the stuck-up dunderhead that he was and suggest he ask Granger, the muggle-born child. How dare he think to question his methods of questioning. Just for that I think I'll see just what he's thinking. Snape thinks and with a mental shove he forces his consciousness towards the boy, but what he finds shocks him.
The boy has natural occlumencly shields! Snape thinks in surprise. There is no way…no way…oh well, I'm sure natural occlumens aren't that hard to come by. He thinks taking house points and yelling at the class to be writing this down. It's not like Dumbledore to let his precious pawn get one over him. Maybe he's been tutored. Yeah, that's it Dumbledors's tutored him...
While in reality it was the furthest from the truth…