Harry James Potter, age seven, was on the school roof. He did not know how he got there, nor did he know that this was perfectly normal (if not a bit premature) for the sort of person that he would have been had his parents been alive. Because Harry's parents were not unemployed, worthless alcoholics who got themselves wrapped around a tree ("And good riddance to bad rubbish!" asHarry's uncle Vernon frequently said). In fact, Harry's mother, Lily, was a witch and his father, James, was a wizard.
Harry Potter was also a wizard. He did not know this at the time, but it is impossible for a wizard to change his nature. As much as his horrid relatives tried to stamp it out of him, he would always have his magic. Untrained, it would periodically take over when he did not mean to use it, such as in the very situation he was in at the moment.
"Okay, I'm on the roof. I must have..." Harry's thoughts trailed off as he realized there was no logical explanation as to how he got there. "Maybe the wind caught me. No, that's just stupid. I'm seven years old, right? I shouldn't think of any freaky explanations about this. There must be something..."
As Harry puzzled over his predicament, something in his mind hit another something in his mind, and his world changed forever. This happens to all magical children at some point in their lives. However, it almost always happens on September first after they turn eleven, in a controlled environment where magic is supposed to happen, with teachers that can undo any accidental damage that can happen. It is not supposed to happen on top of a grimy rooftop of a Muggle building to someone who has not only never even heard of the existence of magic, but assured in the most forceful of manners that it specifically does not exist.
"Why do my relatives always blame me when 'freaky' things happen? Like the time my hair kept growing back, or when my teacher's hair—or the thing she called her hair—turned blue. Why do they always assume it was me? I know they hate me, and I give the feelings right back to them, not that they're ever gonna find that out, but it's a bit ridiculous.
"They always blame these freaky things on me—the freak. Maybe I am causing those things. I need to think about this. But first, how do I get off of this roof?"