"That's my problem," said the man.

He stared at us all at once, somehow, with a look that said that we knew what he was talking about, but we didn't.

"A man leaves Chicago at twelve p.m. on a train heading for Cleveland at sixty miles per hour," he said quickly. "Another man leaves Cleveland at one p.m. heading for Chicago on a train going eight-five miles per hour. At what time will the two trains cross paths?"

One kid, Arush, raised his hand. "Approximately --"

"I know the goddamn answer!"

...

"That's my problem," said the old man, sitting back down in the chair. "I wrote it. That was the one thing I did. The one thing. When you're young you think everything you do is just the beginning. But when you're old, no matter who you are, you realize you only did one or two things."