Little Johnny was walking down the street with his dad...

and he says "Hey, Dad! I'll bet you $5 there's some dogs humping just around this corner!"
Dad considers how likely it would be, and says "You're on, kid!"
Once they get to the corner, they see the dogs going at it and Dad pays up.
So Dad calls Johnnys teacher. "Hey, my kid has this nasty gambling habit, but I can't break him of it because he never loses! Can you help?"
The teacher replies "I'll do what I can"
Next day, Johnny enters the classroom. "Hey teacher! I'll bet you $5 I can guess what color panties you have on!" The teacher demurely declines. All week, Johnny is pestering her about betting on the color of her panties.
On Friday, Johnny repeats the bet. "Hey teacher! I'll bet you $5 I can guess what color panties you have on!"
She says "You're on!" and lifts her dress to reveal she isn't wearing any panties at all!
Johnny pays the money, with much grumbling.
The teacher ecstatically calls his father. "Great news! I made little Johnny lose a bet!"
The father is astounded. "How'd you do that?" he asks.
"Well, Johnny's been bothering me all week about the color of my panties, and today I didn't wear any, so he lost the bet!"
A long string of curses comes over the phone.
Teacher asks "What's wrong? Didn't I do it right?"
Dad answers "Just this morning he bet me $50 he'd see your pussy before the day was out!"