There is something exhilarating about watching a national election pivot on the hinge of a pocket Constitution.

Last month, Khizr Khan, a naturalized American citizen who appeared before the Democratic National Convention with his wife, Ghazala, taunted the Republican presidential nominee Donald Trump by pulling out a well-thumbed pamphlet and asking, “Have you even read the United States Constitution? I will gladly lend you my copy.” The resulting counterattack by the short-tempered nominee seems to have marked a tipping point in the apparent collapse his campaign so far.

Intriguingly, Trump never actually responded that he has read the Constitution. It is apparent from his earlier suggestion that the document contains 12 articles (correct answer: 7) that he probably has not.

As Khan suggested, any American can perform this act of citizenship in a few minutes. And the evidence suggests that many are. Right now, the pocket Constitution is number one on the Amazon list of books about the United States; pocket Constitutions of various editions occupy spots 1 through 5 and 7 through 12 on the “constitution” bestseller list. The American Civil Liberties Union’s pocket volume is now on back order.