It's the old,"A farmer has three chickens that won't lay eggs. So he calls a vetrinarian, and he can't help. So he calls a biologist, and she can't help. So finally he calls a physicist.

The physicist takes copious notes, precise measurements, and spends the rest of the day furiously scribbling in his notebook. Finally the physicist shouts excitedly, and rushes into the farmhouse.

The farmer says, "Did you figure it out?" And the physicist says, "I did! But it only works for spherical chickens in a vacuum."