<em>Weak</em><em> and </em><em>Week</em><em> are </em><u><em>homophones,</em></u><em> they sound the same but have different meanings. Since </em><em>seven days</em><em> makes a </em><em>week</em><em>, the joke (pun) has a couple different meanings as it </em><u><em>relies on a homophone</em></u><em>.</em>