Elaine "Flipper" Hodgson
Have the girls putting on the skit start to build a car - one person is a wheel, seat, steering engine etc. Each couple of people have the engine try to run. The girls in the know, splutter, groan and sound like a dying engine.
Add a few volunteers from the audience - each with a job - the car continues to rattle and groan. Add one last person ( on whom the joke is played) asking them to hold together two "parts". This time the car purrs - turn to the audience and say " just what I thought...we needed one more nut!"
Keep in mind that you should pick people with a sense of humour or the joke can fall flat.