Swedish teenager Greta Thunberg fired up a crowd today at a climate strike in Denver.

As in earlier speeches, the 16-year-old activist chastised world leaders for not heeding what scientists are saying about the climate crisis.

In fact, she accused these politicians of behaving like children.

"They are indeed not mature enough to tell it like it is, so we will do it for them," Thunberg declared.

Some in the crowd shouted "how dare they?" in response to what she was saying.

Thunberg emphasized that young people didn't start going on strike from school because they wanted to do this or because they wanted to gain attention or because it was fun.

"We did it because someone needs to do something," Thunberg said. "And that someone could be me, it could be you, or anyone in this crowd.

"You don't have to wait for someone else to do something," she continued. "No one is too small to make a difference. Never forget that."

You can watch the speech below.