A farmer is having problems with his chickens. All of a sudden, they are all getting very sick and he doesn’t know what is wrong with them.

After trying all conventional means, he calls a biologist, a chemist, and a physicist to see if they can figure out what is wrong.

The biologist looks at the chickens, examines them a bit, and says he has no clue what could be wrong with them. The chemist takes some tests and makes some measurements, but he can’t come to any conclusions either.

Then it is the physicist’s turn. He stands there and looks at the chickens for a long time without touching them or anything. Then all of the sudden he starts scribbling away in a notebook. Finally, after several calculations, he exclaims “I’ve got it!  But it only works for spherical chickens in a vacuum.”