From the New York Times, in all seriousness:

Trump has been urging aides to explore a way to buy Greenland from Denmark, according to three people familiar with the discussions.

His interest in Greenland began last year. At a meeting that spring in the Oval Office, he joked about buying Greenland for its resources, according to a person who was in attendance.

In the year since, the president has repeatedly returned to the topic, asking aides if they can pursue a purchase of Greenland, a semiautonomous territory that Mr. Trump has been taken with in part because of its natural resources, like coal and uranium.

Privately, Mr. Trump’s advisers are highly skeptical that such a move could ever happen. But instead of telling him they do not think it is possible, the advisers have agreed to investigate the matter, according to the people briefed on the discussions.

Yes, just humor him until they can get him to find something else to play with.