"Almost No Difference" joke

"What's the difference between the North American porcupine
and the African porcupine," the society matron asked the
zookeeper.
"The principal difference is the North American species has a
longer prick."
This, as you might assume, distressed the matron who
stormed immediately to the zoo manager's office. The zoo
manager said, "Ma'am, I apologize for my staff's unfortunate
choice of terms. What the keeper should have said is the
North American species has a longer *quill*. In fact, their
pricks are just about the same size."