Chappy is diddling along the motorway when the jam jar suddenly starts stuttering and stalling - he limps over to the hard shoulder just in time before it packs up completely.
Fortunately he's in the AA so he gets on the blower to explain the situation. Details taken, he's told that the AA bloke will be with him as soon as possible. Sure enough, within half an hour the big yellow van turns up and the AA man gets the hood up and starts poking about.
After about five minutes of this, laddo pipes up: "What's up? Can you fix it?"
AA man points with his screwdriver and says: "Shit in your carburettor."
"Oh," says the driver. "So ... how often will I have to do that, then?"