Two things have long obsessed President Trump: Time’s person of the year and the 16-year-old climate activist Greta Thunberg.

This week they collided when Time picked her for its annual cover. It was never in doubt that the president would tweet about it. The only question was when.

Early Thursday, the answer arrived.

“So ridiculous,” Mr. Trump tweeted. “Greta must work on her anger management problem, then go to a good old fashioned movie with a friend! Chill Greta, Chill!”

Ms. Thunberg was quick to respond.

“A teenager working on her anger management problem,” read a new version of her Twitter biography. “Currently chilling and watching a good old fashioned movie with a friend.”