How do it know?

An expression taken from an old, old joke that describes a persons failure to grasp the obvious or demonstrate such intellectual ineptitude as to illicit a comment of " How Do It Know?" The joke refers to a question posed to a scientist by a student ( or in variations IBM executive and employee, Republican and voter..ect...)"Sir in your opinion what is the worlds greatest invention?" asked a student. The scientist replied "The thermos bottle!" "Why?" asked the student, obviously confused. " Well" said the scientist, "it keeps cold drinks cold in the summer and hot drinks hot in the winter." holding up his thermos bottle in awe,"How do it know?"Also see No Shit Sherlock, Duh and George W Bush