The question has perplexed her opponents on the debate stage before. But for the aides who have worked with Clinton in close proximity on campaigns, the hyper-specific and obscure jab, delivered with on-the-spot precision, is their boss’ trademark. As described by current and former staffers, Clinton is a candidate who insists on being “humongously prepared,” who consumes research with “obsessive” rigor — and who, perhaps more than most elected officials, delights in the art form known by political professionals as “oppo.”

The work of opposition research is exacting and unglamorous, carried out by hired hands who spend hours filing Freedom of Information Act requests, monitoring cable news, and sifting through LexisNexis search results. The result is the “book,” a catalog of every possible hit, both savory and not, on a political foe. It can number thousands of pages, and on most campaigns, the candidate doesn’t bother with more than the toplines.

Clinton, meanwhile, has digested something closer to the book itself on nearly every one of her opponents, from O’Malley to Bernie Sanders to Trump. (The New York Times counted “19 pieces of pure oppo” from Clinton in last month’s debate alone. And the second debate on Sunday night, held as Trump faces public condemnation from a decade-old video in which he boasts about groping women, promises more.)