A farmer has problems with his chickens: all of the sudden, they are all getting very sick. After trying all conventional means, he calls a physicist to see if they can figure out what is wrong.
The physicist trys. 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 gruesome calculations, he exclaims, "I've got it! But it only works for spherical chickens in a vaccum."
