Explain xkcd

Date: April 25th, 2011
From: Dan Hoey
Subject: Etymology

StarFreak1138 wrote:

  There's a line in the Star Wars Novelization (ghost written by
  Alan Dean Foster) where Obi-Wan comments, "... even a duck has
  to be taught to swim."  Luke queries, "What's a duck?"

Which in turn is an homage to "why a duck" from the Marx brothers.