Former Republican presidential candidate Jeb Bush mocked President Trump’s plan for a wall along the U.S.’s southern border with a three-word tweet on Friday.

“Reality sets in,” Bush tweeted with a link to a Wall Street Journal commentary piece titled “Trump Can’t Build a Border Wall Without the Real Estate.”

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The piece says the federal government would need to own the nearly 2,000 miles along the U.S.-Mexico border, most of which is currently private property, in order to build a wall.

In order to gain ownership of land it does not own, the Trump administration would need to employ eminent domain, which under the Fifth Amendment allows the government to seize public property for private use with “just compensation." The post says that buying the land could put the price of the wall well above the $10 billion estimate Trump floated during the campaign.

The Trump administration has set its sights on making the border wall a priority.

Homeland Security Secretary John Kelly said earlier this month he wants to complete the wall within two years.