One of the most common class questions in his physics lessons was the numerical calculation of a physical quantity. A favourite was a current or resistance calculation using Ohm's Law. You could, as ever, be singled out for an answer, but giving the numerical answer was not enough, and quite rightly.
The correct units had to be specified as well, for without them the answer was meaningless. An answer of "20" would be met with a hard stare, and the answer repeated back at you. "20. 20 what? 20 elephants?" and the longer the class took to catch on to this and answer properly, the more emphasis (volume) he placed on the word "elephants".
Hence "elephants" became synonymous with missing units in pretty much every question.