Oliver has made a habit on his HBO series of covering obscure policy issues and then pulling off clever stunts to illuminate the problems he’s addressing. There was the time he educated his audience about predatory debt buying companies then bought $15 million worth of medical debt just to forgive it. He called it the largest giveaway in television history. He also started his own church, just to prove how easy it is to get tax-exempt status, during a show about exploitative televangelists.

But, as he noted during the start of last night’s episode, he needed to move away from little-known policy for a moment so that he could examine a larger, trickier topic: reality. The episode was a deep dive into the falsehoods the president and his team have propagated.

“[Trump] even lied about the weather at his inauguration,” Oliver said with his signature incredulity. “We have a president capable of standing in the rain and saying it was a sunny day.”

Any policy decision needs to start with a shared sense of reality, Oliver said, so he devoted his episode to answering questions such as: “How did we get a pathological liar in the White House?” Basically, he wanted to break down how bad information spreads, why people believe it and what the general public can do to combat fake news.

The pattern Oliver has seen with Trump has been that the president “sees something that jibes with his worldview, doesn’t check it, half remembers it, then passes it on.”

It all starts with the sources of the president’s information — places like Breitbart and Infowars. According to Oliver, for anyone who also gets their “news” from those places, “Trump looks like the first president ever to tell the real truth.”

John Oliver on “Last Week Tonight.” (Eric Liebowitz/HBO)

As always, the comedian explained the issue with some funny detours along the way, reminding viewers how tenacious rumors can be with little more than a picture of Richard Gere. He also said that, “faith and facts aren’t like Bill Pullman and Bill Paxton. When you confuse them, it actually matters.”

So what can people actually do about the spread of bad information? He said social media users should be savvier about what they read and share on the Internet — even if it’s a story that confirms their own views.

And, of course, Oliver is doing his part with his ad campaign, which debuts this morning between 8:30 a.m. and 9 a.m., according to the host. The cowboy isn’t just going to explain global warming. He’s also enlightening viewers — and one in particular — that killing a terrorist’s family is a violation of the Geneva Conventions; not all black people live in the inner cities; and the definition of the nuclear triad.

“Just remember, Donald,” the cowboy says to the camera, “if you don’t know, it’s okay to ask.”

In an interview with the New York Times last week, Oliver said that he wasn’t going to spend his whole season on the president. So many people are covering Trump that it’s “a lot of people feeding on the same carcass,” he said. Still, he had to address the new administration somehow, and, no surprise, he found his own unique way of doing it.