The Muffler

A blonde got a dent in her car and took it in to the repair shop. The repairman, noticing that the woman was a blonde, decided to have a wee bit of fun. So he told her that all she had to was take it home and blow in the tailpipe until the dent popped itself out. After 15 minutes of this, the blonde's blonde friend came over and asked what she was doing.
"I'm trying to pop out this dent, but it's not really working."

"Duh. You have to roll up the windows first!"