And then, at 8:25:50 pm ET, the president retweeted an account he had never retweeted before. The account had posted a video of former Vice President Joe Biden, crudely and obviously manipulated to show him twitching his eyebrows and lolling his tongue. The caption read: “Sloppy Joe is trending. I wonder if it’s because of this. You can tell it’s a deep fake because Jill Biden isn’t covering for him.”

Whatever the intentions of the original tweeter—it purports to be the account of a left-wing activist supportive of the candidacy of Bernie Sanders—the Trump retweet looks like an experimental test of the rules of social media. Since earlier this year, Twitter has banned images that have been “significantly and deceptively altered or fabricated,” especially if they are likely to cause serious harm in some way. Because the account retweeted by Trump explicitly labels its image a “deep fake,” it arguably does not violate Twitter’s anti-deception policy. As of 8:30 this morning, the image remained live on Twitter and present on pro-Trump Facebook accounts.

In the campaign of 2016, Trump benefited from fake news disseminated by troll accounts, some Russian-sponsored. The most circulated fake-news item of 2016 claimed that Pope Francis had endorsed Trump. That item was unlikely on its face. Pope Pius XI did not endorse Al Smith in 1928. Pope John XXIII did not endorse John F. Kennedy in 1960. Donald Trump launched Twitter attacks on the present pope in the spring of 2016, after the pope rebuked Trump’s stance against immigrants and refugees. But the power of a fake-news item does not depend on its plausibility. The power of a fake-news item depends on its target audience’s will to believe. In the autumn of 2016, many culturally conservative white Catholics strongly wished to believe that voting for Trump was consistent with their faith. Somebody provided them with a basis for that desire to believe, and the invitation was eagerly seized.