A farmer was having terrible problems with his chickens - they were becoming quite ill, so he called in the local vet. When the vet arrived, he examined the chickens, but was unable to determine what the problem was.
The farmer thought, "Let's see what the more traditional sciences can tell me", and called the local university who sent out a biologist, a chemist and a physicist.
The biologist went first - she looked at the chickens feathers, and studied some of their cells under an optical microscope - all to no avail, she couldn't work out what the problem was.
Next, the chemist mixed up some elixirs using his portable test tube rack, took a little of the chickens' skin, and performed a few exothermic reactions, but also had no idea of why the chickens were sick.
Finally, the physicist arrived.
He insisted that he didn't need to see the chickens, but requested a whiteboard and some markers.
For about twenty minutes he derived equations, drew graphs and mumbled to himself, until finally he yelled, "I've got the solution!"
"Great!", exclaimed the farmer, "That's fantastic!"
And then the physicist said.
"But it only works for spherical chickens in a vacuum..."