I didn' Peace so I looked it up and thought I'd post it for the benefit of other depraved deprived Mudcatters.

Mr Opperknockity was the best piano tuner on earth, often he was booked months in advance. One customer, who had waited half a year for Opperknockity to tune his 9' Steinway, called him a few days after the tuning; "Really, everything sounds wonderful...except that middle C has slipped a little. Can you come and touch it up?" "No, I can't do that" replied Opperknockity. "But why not?" said the indignant customer. "Because, everyone knows Opperknockity only tunes once"