What did Donald Trump do today?

He angrily rebutted his own chief of staff's comments about his position on the border wall.

Yesterday, John Kelly said in a televised interview that candidate Trump's position on the Mexico border wall was "uninformed" and that since taking office, Trump's position had "evolved."

Furious, Trump took to Twitter to deny any "evolution" of his wall, and told reporters that Kelly simply hadn't said any such thing--or at least that when Kelly said exactly that, it wasn't what he meant to say.

In fact, Trump has taken any number of positions on the wall, depending on who he thinks is listening. On the campaign trail, Trump spoke vividly of an "impenetrable, physical, tall, powerful, beautiful, southern border wall." But since the election, he has been describing something most people would call a fence, with gaps in it. In the absence of any Congressional desire to spend money on the project, even renovation of the existing fences Trump seemed to think were so inadequate has counted as "The WALL" for bragging purposes.

On the question of funding the wall, Trump's position has "evolved" even more. After the election, he began retreating immediately from his call-and-response claim that Mexico would pay for the wall. According to a leaked transcript of a secret phone call with Mexican president Enrique Peña Nieto in January of 2017, Trump acknowledged the Mexican government's position that it would never pay for anything, and simply asked Peña Nieto not to embarrass him by saying so to the press. (Trump also called the wall "the least important thing" in that call.) In August, Trump threatened to shut down the government unless Congress appropriated American taxpayer money to pay for building some of it--a threat he repeated today.

Why does this matter?

• There's nothing wrong with a president's positions changing, but there is something wrong with a president who gets angry because a staffer trying to help him pointed out that they had.