How big is two by four construction timber?

A urban legend says that an MIT professor has asked, as a bonus question, on the first test of the year, for almost 40 years, the question; "What is the width and height of a two by four construction timber?" The legend says that the failure rate (wrong answer) for that question went over 50% in 2005 and has stayed has stayed over 50% since then. Then they ask what is wrong with the world today. :)

By the way, 2 by 4 construction lumber should be 2 inches thick and 4 inches wide but is usually slightly less, down to 1.5 by 3.5 inches.