White Oak

I just started getting The Big Bang Theory reruns on TV.  Listening to it last week, one of the characters gave a joke at the beginning of a Physics Conference.  I thought it was so funny I told it to my friends this weekend.  They didn't see the humor (and explaining a joke never works).  So... did I tell it wrong or is it kind of a physics thing?  You tell me if it's funny or not.

A farmer's chickens stopped laying eggs so he asked his friend, a physicist if he could figure out why.  The physicist worked on it for awhile and then told the farmer, "I've got it but it only works in the case of spherical hens in a complete vacuum."