In 1951, Sen. Hubert H. Humphrey warned public school teachers against discussing “controversial issues” that weren’t controversial at all. “I know from my own teaching experience how much heat is expended in classrooms when the debate rages over a fact as if its existence were a matter of opinion,” wrote Humphrey, a former political science professor and future vice president.

At the same time, Humphrey acknowledged that teachers often faced pressure from local citizens “who prefer that only ‘truths’ be taught — which they define as beliefs approved by them.” So teachers also had to resist the impulse to present opinions — either the community’s or their own — as unquestioned facts. And most of all, they had to teach students how to distinguish between the two.

That’s always been easier said than done. But it’s probably never been harder than it is in the era of “fake news,” when lies can encircle the globe with a single click of a mouse. Hillary Clinton ran a child prostitution ring out of a Washington pizzeria. Barack Obama signed an executive order banning the Pledge of Allegiance in schools. And so on.

Then there are the falsehoods perpetuated by our fact-challenged commander in chief, Donald J. Trump, who launched his political career by claiming that Obama wasn’t born in the United States. Other presidents have twisted the truth, of course. But Trump has taken that to a whole other level, blithely misrepresenting facts with head-spinning frequency.

One of the things Trump misrepresents is fake news itself, using the term to dismiss any report he doesn’t like. So we need to teach our students how to identify wholly false news stories by their tell-tale signs: obscure URLs, unknown authors and so on. ...