CBS late-night host Stephen Colbert mocked President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE on Tuesday night over a quote in veteran journalist Bob Woodward's forthcoming book comparing the president to a "fifth- or sixth-grader."

Colbert pointed to an excerpt of Woodard's book about the Trump White House, which quotes Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE discussing Trump's understanding of the situation on the Korea Peninsula.

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“Secretary Jim Mattis struggled to convince Trump that troops should stay in South Korea. Afterwards, Mattis told associates ‘that the president acted like — and had the understanding of — "a fifth- or sixth-grader,"'” Colbert said, quoting Woodward’s book.

Colbert, a frequent critic of Trump, joked about creating a new game show he dubbed "Are You Smarter Than The President?" He said the fictional game show would be a "show where everyone’s a winner."

Mattis on Tuesday denied having said the words attributed to him in Woodward's book, calling it “fiction” and “a product of someone’s rich imagination.”

“The contemptuous words about the President attributed to me in Woodward's book were never uttered by me or in my presence,” Mattis said in a statement.

“While I generally enjoy reading fiction, this is a uniquely Washington brand of literature, and his anonymous sources do not lend credibility,” he added.

The president also blasted the book on Wednesday, asking why politicians don’t change libel laws to punish what he says is a work of fiction.

“Isn’t it a shame that someone can write an article or book, totally make up stories and form a picture of a person that is literally the exact opposite of the fact, and get away with it without retribution or cost,” Trump tweeted.

“Don’t know why Washington politicians don’t change libel laws?” Trump added.