Via the November 13, 2006, episode of The Writer's Almanac, a poem of the same title by Ron Koertge in Fever:

Kryptonite

Lois liked to see the bullets bounce
off Superman's chest, and of course
she was proud when he leaned into
a locomotive and saved the crippled
orphan who had fallen on the tracks.

Yet on those long nights when he was
readjusting longitude or destroying
a meteor headed right for some nun,
Lois considered carrying just a smidgen
of kryptonite in her purse or at least
making a tincture to dab behind her ears.

She pictured his knees giving way,
the color draining from his cheeks.
He'd lie on the couch like a guy with
the flu, too weak to paint the front
porch or take out the garbage. She
could peek down his tights or draw
on his cheek with a ball point. She
might even muss his hair and slap
him around.

"Hey, what'd I do?" he'd croak just
like a regular boyfriend. At last.