New shoes

A guy buys some shiny shoes and notices that when he stands near a girl he can see up their skirt! So on Saturday night he goes to the local dance. When his girlfriend arrives he says, “You’re wearing blue knickers tonight.”
“Yes,” she says a bit surprised. “How did you know?” He explains her about the shinny shoes. With a mischievous look she says, “Why don’t you check my sister?”
The guy goes to ask for a dance, but when he gets up close he staggers back in shock and has to take a seat.
“Well,” says his smirking girlfriend, “I see you’ve noticed that my sister doesn’t wear knickers!”
“Thank go for that,” replies the young man, “For a minute there, I thought one of my shoes had a crack in it!”