Perfectly Spherical Cows

A farmer had a cow that wouldn't give milk. He called the vetranarian. The vet checked the cow's vitals, her feed, her living conditions, and then told the farmer "I have no idea what's wrong with your cow. But I'm going to call my friend, a professor at the Ag school. He'll know what's wrong."

The professor arrives at the farm. She reviews the vet's notes, takes blood and saliva and stool samples and checks them under a microscope. She also gets samples of soil from the pasture. But after some initial labs, she says "I don't know what's wrong with this cow. It seems perfectly normal to me. I'm going to call another professor, a physicist. He doesn't know anything about farming, but he's very smart and solves weird problems all the time."

The physicist arrives at the farm. He measures the barn and the pasture. He asks the cow a few questions, then scribbles on a notepad. Finally, he looks up and shouts, "I've got it! Now, assuming a perfectly spherical cow of uniform density...."

This is about economists, not physicists. But I couldn't come up with an economist version of the joke.