In a little over a month, the man who claimed he would build a wall along the border between the US and Mexico will be sworn in as the new president of the United States. But can Trump really build a wall?

Basically? Nope. And here’s why.

First of all, Trump doesn’t have the power to build a wall as President of the United States. Most executive power comes from the military executive actions and the fame that comes with being the figurehead of one of the most powerful and influential countries on Earth. But that’s about it. The president, believe it or not, doesn’t have the power constitutionally to order a great big wall to be built between us and our third-largest trading partner.

According to the Office of the US Trade Representative, last year our total goods and services trade with Mexico was about $583.6 billion. So what will happen if Trump decides to put up a giant wall between us and them, and attempt to make them pay for it? Almost certainly, they won’t pay for it, so we will end up settling the tab through more taxes.

And that tab will be a bit more than you’d think.

In an interview with NBC, Trump stated, “The wall is going to cost …maybe 10 or $12 billion, and it’s going to be a real wall. It’s going to be a high wall. It’s going to be a beautiful wall.”

And he hasn’t actually been very clear. In an earlier NBC interview, Trump claimed that the wall would cost $8 billion, and that it will be made of “precasts [of concrete] going up probably 35 to 40 feet up in the air.”

So is it $8, $10 or $12 billion? Either Trump doesn’t know what he’s talking about or the price of border walls went up drastically since his previous statement.

But this is all old news, right? In the three weeks since he won the election, President-elect Trump has changed his tune about five of his biggest promises during the campaign, according to Newsweek. One of those changes was about—you guessed it—the border wall.

Less than a week after the election, Trump stated in an interview on 60 Minutes that he would only build a wall “for certain areas” along the border, according to Newsweek. That’s a bit contradictory to his original statement.

The point is, our soon-to-be new president almost certainly won’t build a wall on the border between the US and Mexico like he promised, and that’s due in part to the fact that he can’t—at least not in the way he described.