As told by Mr. Karl Sinfield in the comments section of a Tom Chivers blog post at the Telegraph:
A geneticist, a physiologist and a physicist were summoned to meet a wealthy racehorse magnate. He told them he would give a million pounds to the one who could accurately identify race-winning horses. After six months of hard work, they returned to present their results to the expectant millionaire.
The geneticist said, "I've looked into all the current genetic research, checked blood-lines going back decades, but there are just too many behavioural and environmental factors. I can't help."
The physiologist said, "I've looked at muscle mass, bone volume and density, and all the other factors I can think of, but the problem's too complex. There's just no guarantee of predicting a winner."
Finally, the physicist calmly walks up to the millionaire and gives him an index card. "Here you go," he says "I've found an equation that solves the problem for you."
"Wow," said the millionaire, "That's impressive...I'll get my cheque book."
"Great. But there's one thing you should know," said the physicist. "It only works for a spherically symmetric horse travelling in a vacuum."
(Via Martin Robbins)