This is an excellent joke to spot scientists in an audience. They are the ones who laugh.
A dietician, a geneticist and a physicist are arguing over how to get a horse that's good at winning races.
'It's easy,' says the dietician. 'You just feed it a carefully balanced and monitored diet.'
'No, no,' says the geneticist. 'It's all a matter of breeding for the best attributes for racing.'
'Hmm,' says the physicist. 'Let's assume the horse is a sphere...'