I'm not sure which was the first story of his I read -- "A Sound of Thunder" (quite possibly, since it had dinosaurs, so was the sort of thing I would have searched out) or "All Summer in a Day" (which did not have dinosaurs, but which a dim memory suggests was part of a short story collection assigned at school.)
I do know that both of these stories, and others, entered my imagination and remained there. It's been years since I've read either, but I can still remember the endings, and how awful and devastating and real they felt, how I learned the power of harsh endings. I can think of very few other works that I read at eight that I can still remember so well today.
Years later I saw him at a DragonCon (1998). I was with two randomly met guys at a con; he was talking with a couple of other authors. We talked about going up to speak to him -- this was just before DragonCon's real crowds, and it might have been possible -- but we chickened out. I went to one of his panels instead, but still didn't manage to talk to him. Still, I'm glad I saw him the once.