What's the difference between a BMW and a porcupine? Unlike the porcupine, the pricks are on the inside.
BMW vs Porcupine
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Explanation
This is a pun using the word "prick", which can mean a sharp point as on a porcupine, but also an unpleasant or annoying person. The joke implies that the people who own or drive BMWs are often considered unpleasant.