This is one told on "The Big Bang Theory" by Sheldon as he prepared to address a group of students:
A poultry farmer was distressed because his chickens stopped laying eggs. He didn't know what to do about it, but one of his friends, a physicist, offered to help. The physicist came out to the farm, took a bunch of measurements, and went back home to analyze the data. The physicist called the farmer on the phone and said: "Okay, I have a solution for your problem, but it only works with spherical chickens in a vacuum."
If anyone gets a giggle out of this, please let me know why!