Before deriding someone, it is generally advisable to make sure you are not about to immediately open yourself up to that same derision. For example, if you are going to make fun of someone for pronouncing a word incorrectly (snob), make sure that you do not then go on to misspeak. For another, if you are going to tease your partner for forgetting a grocery item, be certain, on your next trip to the store, not to neglect to pick up the seltzer. For a final example, if you are running for president and wondering aloud on social media how on Earth your opponent doesn’t know what it means when an email is marked with C, make sure you yourself actually know. Per CNN:

The FBI had released a report Friday that detailed its investigation of Clinton’s email use at the State Department along with notes from the bureau’s interview with Clinton. According to that report, Clinton told the FBI she was unaware that the “c” markings in certain emails denoted the presence of confidential information.

Donald J. Trump, Republican candidate for president, asked the following on Twitter:

Lyin' Hillary Clinton told the FBI that she did not know the "C" markings on documents stood for CLASSIFIED. How can this be happening? — Donald J. Trump (@realDonaldTrump) September 4, 2016

Good, if dramatic, question. For the answer, America and this CNN article turn to John Noonan, national security adviser to Jeb Bush and Mitt Romney.

(C) stands for CONFIDENTIAL. Confidential, Secret, Top Secret are levels of classification. DoD Directive 5200.01. https://t.co/j3jzMrIg5z — John Noonan (@noonanjo) September 5, 2016

How can this be happening?

Read more Slate coverage of the 2016 campaign.