Thumb Sucking

Johnny gets up in the middle of the night to get a drink. His little brother Billy hears him and gets up, too. On the way to the kitchen, they hear noise coming from their parent's bedroom and decide to peek through the keyhole. Johnny goes first. "Oh, my!" he says, backing away.
Billy takes a look. After a few seconds, he says angrily, "I can't believe it."
What? What can't you believe" asks his brother.
"I can't believe Mom has the nerve to yell at me for sucking my thumb!"