(Dirty) In person/group joke.

Works better with a small to medium group:
SO there was this 18 wheeler driver going down the road one day and he saw this person on the side of the road. He pulls over to offer assistance. They come up to him and ask him for a ride. The truck driver not really wanting to help out the hitch hiker thinks for a second. "I just need to get to (name a location that is only a few miles away from your location at the moment.) and I am really late." He hesitantly agrees. After a few minutes on the road, the hitch hiker says "I cannot thank you enough for this, I don't have any gas money, but would you accept a blow job? The truck driver (not very attractive) agrees. Right before they get to the turn, the truck hits a really big bump and while the hictch hiker is almost done, they hit their head on the, the um (act confused pretending to have a brain fart and motioning trying to remember until someone says dashboard)[then look up and say] "how do you know?".