In a campaign that has created a cornucopia of wonderful, GIFable moments, this is still one of the better ones.

That's Donald Trump in Iowa on Nov. 12, showing how floppy his belt was as a way of undermining the story from Ben Carson's biography in which Carson says his attempt to stab a friend was foiled when his knife hit a belt buckle. Look how much belts move around! Therefore, Trump suggests, Carson's story can't be true.

In an interview with The Post published on Wednesday, Trump says that it was that moment which doomed Ben Carson. When Trump exercised his belt, Carson was at 24.4 percent in Real Clear Politics's average of polls -- second place, and only 0.4 percentage points behind Trump. Today, Carson is at 13.8 percent and has dropped to fourth.

"You had to see the whole speech," Trump told our Steven Ginsberg and Robert Costa. "It was so good. That was when Carson went down. That was the thing that totally took him down. In Iowa, big audience."

Ginsberg pushed back. "Don’t you think it was his lack of knowledge on foreign policy?" he asked, a reference to the generally accepted idea that Carson's slide has suffered in the wake of the terror attacks in Paris.

"No," Trump replied, "that may have been the beginning of the end, but my speech is when he really began to go down."

Setting aside the contradiction in that last sentence, the assertion is hardly atypical bluster for Trump. He repeatedly took credit for ousting former Texas governor Rick Perry from the race and for submarining Sen. Lindsey Graham, although that wasn't the case.

So is it true? Was Trump's belt-wiggling the beginning of the beginning of the end?

One bit of data that's important to mention that we've left out for effect: The belt-wiggling happened about 24 hours before the Paris attacks. In polls conducted after the attacks (and the wiggling), Carson slipped. No polls were in the field during that period.

Without asking people who've changed their minds why they've done so, we're left to interpret the poll data we have at hand to suss out the reasons for any change. We know that Carson was not a top pick on foreign policy or fighting terrorism prior to Paris; as the issue rose to prominence among voters afterward, Carson fell. But we can't say definitively that Trump's gyrations weren't the cause of Carson's downfall, any more than we can say definitively that the missing Malaysian plane wasn't spirited off to Mars for various alien experiments.

So Trump says it was thanks to him. And, if history is any guide, I can expect comments and tweets asserting that actually, it was, because of reasons. The 2015 Donald Trump campaign in a nutshell.