A man moved to another state where he didn't know anyone. In the move, his old piano was jarred, and of course it needed to be tuned when the man arrived.
So he asked around, and was told that Earl Opporknockity was the best piano tuner in the area. The man called Earl and hired him to tune his piano.
Earl had a keen ear and a deft touch, and did a wonderful job tuning the old piano. The man was able to play beautiful music once again, and was very pleased.
After a year or so the old piano started producing sour notes again. So the man called Earl, and asked him to come work his magic on the old piano again.
To the man's surprise, Earl refused, saying "Sorry, I can't accept the job."
"Why not?" the man wanted to know. "I'll pay you twice as much as last time if you'll just come tune my piano."
"Haven't you heard?" Earl asked, "Opporknockity tunes but once."