Paddy - "I've been trying to sell my car for weeks, but no one want's to buy it"
Murphy - "How many miles has it done?"
Paddy - "109,000 miles"
Murphy - "That's very high, why don't you wind the clock back?"
Paddy - "That's a good idea"
A couple of weeks later, Murphy sees Paddy in the pub and asks how he got on selling his car. "Why should I want to sell it now" says Paddy. "It's only done 49,000 miles"