So there's this guy who owns a dairy farm. He sets his cows out to pasture and brings them in for milking every day. Back when his great-grandfather owned the land, the pastures ran right up to the barn but in the days since a road was paved, separating the fields from the barn. In the morning, Farmer Dave milks the cows and then takes them across the road. At night, he brings them back.

One day he goes out at the usual time to the pasture to bring the cows back across the road. Dave brings the cows up to the edge of the road but for some reason, the cows won't go across. He tries everything he can think of and still they won't step onto the road. Afraid that the cows might be stolen or eaten in the night, Dave sleeps out in the pasture with them.

The next morning, a big city lawyer on his way to work comes driving along the road and sees Farmer Dave looking a little worse for the wear.

"Looks like you need some help," he says.

"Sure do," says Dave, "I just can't seem to get my cows back across the road to milk them."

"No problem!" says the lawyer, "I know just how to fix that." He leans over into the glovebox of his car and pulls out a large stack of papers covered in small writing.

"Fill those out and mail them to the USDA and they'll send someone to straighten these cows out." With that the lawyer drove away. Well Farmer Dave had no other choice so he filled out the papers and mailed them but no one ever came.

A week later Dave is still spending the nights out in the fields. A doctor in his fancy sports car pulls off on the side of the road:

"Looks like you're having some trouble with your cows."

"No kidding," says Dave, "They refuse to go back across the road."

"Try these," says the doctor as he hands Dave a bag full of pills. "They're sedatives, they'll put your cows right to sleep and then you can carry 'em over." So Farmer Dave tries feeding the pills to the cows but they must have been expired because not one cow fell asleep.

A month later Dave is looking pretty grizzled from being out with the cows. His clothing is covered in mud and he needs a shave. Puttering over the hill comes a small sedan, barely making it up the steady incline. With a shudder it comes to a halt in front of Farmer Dave. The physicist inside rolls down the window and squints at Dave through the haze and the morning glare.

"Top of the mornin' to ya."

Dave, a little perplexed by the strange turn of phrase stammers a bit: "I, uh, I got these . . . um, my cows . . ."

The man interrupted "Hey, I think I know how to do this, let me get back to you!" and speeds off, car coughing out thick blue-black oil smoke.

The next day the same car comes driving up again and stops in front of Dave. The physicist gets out of the car and hands Dave a binder full of sheets with complex equations written on them. "I've got the perfect solution for you but it only works for perfectly spherical cows in a vacuum."

If you don't get it, get thee to a physics class!

The IDFA wanted to find a way to make cows produce more milk. So they decided to consult with the foremost biologists to find ways to improve production. After five years of research, 20 million dollars spent, hundreds of protests by Greenpeace and PETA, they came back with a New and Improved Cow. It had a milk production improvement of 5% over the original.

The IFDA was somewhat disappointed by these results. They decided to try again, so this time they engaged the greatest chemists in the world. After two years, 10 million dollars spent, and the accidental poisoning of a small town in Colorado, they got a 3% improvement in milk output.

The IFDA was again underwhelmed by this success, but undaunted they decided to try hiring the greatest physicists available. The physicists tried for a year, and after spending 5 million dollars on radiation therapy and centrifugal distillation of cattle forage, they got a 0.1% improvement in output.

In desperation the IFDA turned to the mathematicians. Upon hearing the problem, the foremost mathematician of our time offered to solve their problem for a marginal fee. He told the delegation that they could come back the next morning and he would have the solution for them. The next morning they returned, flushed with anticipation, and he handed them a piece of paper with the computations for a 300% increase in milk production.

The paper began: "Consider a perfectly spherical cow, radiating milk isotropically".

This joke has appeared in many different forms, generally mocking the profession of either the mathematician or the physicist. Variations include jokes calling for a 'perfectly spherical racehorse', 'perfectly spherical chicken, or even the ever humorous 'perfectly spherical Jesus'. (No, I'm not going to post the Jesus joke. It's a little too derivative.)

Q: How many theoretical physicists does it take to change a lightbulb?
A: Just one, but he can only do it if it's a perfectly spherical lightbulb.

The reason that this is 'funny' is that many calculations work much better of you can assume that a star, volume, or cow is a perfect sphere. Mathematicians and physicists realize that this is not always practical, and have taken up the phrase "perfectly spherical cow" as a sort of self-mocking mantra. When talking about a highly simplified hypothetical model of reality you will often hear a joking reference along the lines of "...of course, that assumes perfectly spherical cows".

A biologist, a chemist and a physicist are shown a cow, and asked to describe what they see. The biologist goes first, and says "Well it's a Bos bovis part of the Bovidae family, in the mamalia class, etc."

The chemist goes next, stating "I see a complex assembly of organic molecules, assembled into proteins to form a self-sustaining and replicating system..." And on he goes, describing the wonderous chemical interactions in front of him.

When he's finally finished, the physicist says "Well, in a first-order approximation, it's a sphere."

You might well wonder how this joke could possibly have so many forms? Surely one is enough? Well, there aren't nearly as many math jokes as you might expect, so the ones that exist get stretched a little thin. These three jokes are an excellent indicator of how nerdy you are. If you laughed at the first one, you can get along with a nerd. If you laughed at the second one, you are a nerd. If you were still laughing at the third one, it's time to step away from the computer and get a life.

Now if you'll excuse me, it appears that I need to go get a life.

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