Aren't all jokes linguistic?

Revisiting double negatives: this time in joke form. Two people told me this joke today. One attributed it to Garrison Keillor; the other attributed it to a Purdue professor. My guess is Keillor told the joke this weekend and it is now making the rounds in English and linguistics departments.

A linguistics professor stood in front of his class and began his lecture on double negatives: "In the English language it is well known that two negatives equal a positive. This is not the case in every language. In some languages two negatives equal a negative. There is however not a single known language in which two positives equal a negative!"

A cynical student in the back row crossed his arms and muttered "Yeah right."