Corny Joke

One summer evening during a violent thunderstorm a mother was tucking her small boy into bed. She was about to turn off the light when he asked with a tremor in his voice,
"Mommy, will you sleep with me tonight?" The mother smiled and gave him a reassuring hug.
"I can't dear," she said.
"I have to sleep in Daddy's room." A long silence was broken at last by his shaky little voice:
"The big sissy."