TORONTO -- U.S. President Donald Trump seemed to mock 16-year-old environmental activist Greta Thunberg Monday night on Twitter, after a piercing glance from the teen at the UN climate change summit went viral.

If looks could kill,” wrote American singer and actress Bette Midler, who has been a vocal opponent of Trump on Twitter.

A short clip shared widely online shows the Swedish teen watching Trump with furrowed brow as he enters a room. Many users have dubbed it a “death stare.”

Earlier in the day, the teen made an emotional speech, which has been viewed millions of times online. “You have stolen my dreams and my childhood with your empty words,” she said. “People are suffering. People are dying. Entire ecosystems are collapsing. We are in the beginning of a mass extinction and all you can talk about is money and fairy tales of eternal economic growth. How dare you.”

In response to the speech, Trump tweeted a mocking message about the teen: “She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!”

Thunberg returned the mockery and turned the tweet into her Twitter bio, which now reads: “A very happy young girl looking forward to a bright and wonderful future.”

But the so-called “death stare” dominated much of the Twitter conversation after Thunberg’s speech, many tweeting support for the teen.

“I wish your death stare was accompanied by real laser beams,” wrote one user. “Greta Thunberg staring straight into Trump's dark, cavernous soul,” wrote another user.

Others memed the moment further, putting the word “Asteroid” over Thunberg’s face and “Dinosaurs” over Trump’s.