The joke is that “there are more molecules in a single glass of water than stars in the galaxy” is a common thing to say. So the reader immediately sees “hydrogens in a single molecule of water” and assumes that the writer of the statement has made a mistake. It can take a few readings before noticing that the writer said “stars in the solar system” and not the expected “galaxy”, the writer was correct all along, it is us who is the fool. Such fun.
