Sirius Black and James Potter had their first encounter on the platform of nine and three quarters. In the hazy smoke the two had crashed into each other, said "watch it!" at the same time, and walked away with smirks on their faces. James caught a glimpse of unruly hair. Sirius saw the glint of a pair of glasses. They thought nothing of the encounter, completely unaware that it was the beginning of their unbreakable friendship.
They next, inevitably, saw each other on the Hogwarts Express. Sirius had discovered an empty compartment in the distance and headed straight for it. James had a similar idea, and neither was aware of the other aiming for the same door. James was coming from the left, Sirius from the right. Their sides collided.
"Hey, careful, this is my good side!" Sirius shouted.
"You have such bony hips!" James yelled.
Sirius gave James a dirty look and entered the compartment. "Yeah, well your hips aren't doing you any favours either," he grumbled. James either didn't notice or chose to ignore him. Later, Sirius would decide that James simply didn't hear him, for he knew that James didn't back down from an insult.
James sat with his left side against the window, Sirius with his right (and his 'good side' to the door). Sirius gave James an approving nod and introduced himself. "Sirius Black," he announced. He judged that James had a good character, seeing as he didn't scrunch his nose up at the sound of Sirius' name. "James Potter." Sirius grinned and looked out of the window.
"Potter, eh?" he mused. James looked at him.
"What's wrong with being a Potter?" he demanded.
"Nothing," Sirius assured him quickly. "But my parents would be horrified."
James looked down in disappointment, but upon seeing Sirius' grin, he brightened. "Same, with you being a Black and all." When he saw Sirius was not offended by his insulting joke, he felt a kindred spirit in Sirius. He opened his mouth to make another joke but was interrupted.
For at that moment, the most beautiful red headed girl he had ever seen and a greasy looking boy who was dull and disgusting in comparison to her beauty entered the compartment and sat. James and Sirius looked at each other, feeling an instant dislike for the boy who sunk in on himself and stuck his big nose into a dirty book. James couldn't keep his eyes off the girl, however, and he was dumbstruck by the way she was staring at him uncomfortably.
"Where are you heading, if you've got the choice?" Sirius asked, blatantly ignoring the two arrivals.
James shook his head. "Uh… 'Gryffindor, where dwell the brave at heart!' Like my dad."
And so the two purebloods – although their blood status never came up in conversation because neither cared – bonded on the train with their hatred for the greasy kid and their witty banter. They were but eleven years old and it was the beginning of a friendship that would last a lifetime.