A gynaecologist is sat at home one night when a leaflet for local night school classes pops through the door.
Having no hobbies he has a look through it and decides to have a go at motor cycle mechanics.
At the end of the term there's three of them left on the course and they have to do an exam where they rebuild an engine observed by an examiner.
The first guy goes in and puts it all back together, "that's perfect, 100%," says the examiner.
The second one goes in and it's same again, "perfect, 100%."
Then the gynaecologist has his turn. When he's finished the examiner says, "top class, that's 105%."
The other two guys turn round and say, "hang on a minute you said we were perfect, why does he get 105%?"
"Ah," the examiner says, "the extra 5% is for doing it all through the exhaust pipe!"