Who says Mexico can't pay for wall?

Why are critics and pundits so quick to dismiss Donald Trump's claim that he (we) can make Mexico pay for "the wall"?

In 2014 we sent over $971 million in "aid" to Mexico. Perhaps Mexico might use some of that money to fund the building of the walll; or perhaps we just use that money to build the wall.

Martin Dettmer

Wheaton