The spherical cow joke generally goes that a farmer has some problems with his cows (maybe it’s how much milk they’re producing I don’t remember) and so his daughter says “you should ask my boyfriend to help - he’s a physicist and really clever”. So the farmer asks the boyfriend and he says “Well, assume the cows are spherical…”

The joke being because when you do mechanics you generally start modelling any problem with a lot of simplifying assumptions. In particular, that certain things are particles- spherical and uniform.