Ooooh, I know this story very well, in fact I'm just re-reading! It's "Trading places" by Irene Dutch (Very good choices Wendy and AnKit!)

A new one... just a moment...

Ok, I'm going with a favourite of mine:

Quote
"Why have you got Superman's bandage on your shoulder?" Lois
studied the face in front of her...she reached up and pulled his glasses
off his face. Those eyes...slowly she pushed the hair back from his
forehead, while he continued to stare anxiously at her with those wide
brown eyes of his...of-

"Superman's. You're Superman." She snatched her hand away from
his hair.

"I'm sorry, Lois. It wasn't meant to happen like this." He was
supposed to tell her when the moment was right, when he had it all figured
out and could explain it so that she would understand everything. Not this
way, not by accident.

"Why were you pretending to be Clark? Where is he?"

For a split second, Clark considered taking advantage of her
confusion, of perpetuating the deception, but just as quickly he knew that
he didn't have a plausible answer for her question, and anyway, he was
tired of lying to Lois.

"I wasn't pretending to be Clark. I *am* Clark; I'm just Superman
as well."
Good luck!

Simona smile