Cattle, cars and firearms
Over on Language Log, Bill Poser presents an entirely rational argument for concluding that a cow is not a motor vehicle. I would just like to note, however, that there are genuine laws of the land according to which cows and cars do form [oh, fine, belong to...] a natural class, namely the class of items which you can be charged with 'Grand Theft' for stealing one of [e]. See, if you dare, my post on the topic from last year.
Update: See follow up posts on A Roguish Chrestomathy, p(b)ilokok, and the Language Log! Everybody's into categorization.