Get ready to see #climatestrike everywhere today.
It is connected to a strike that calls upon students all over the world to leave school in order to persuade governments to do something about climate change.
Strikes are taking place today in more than 2,000 towns and cities, in over 100 different countries
Thousands of students in the UK are taking part, with strikes in Northern Ireland, Wales, Scotland and England.
But many people are unhappy that students are skipping school to protest.
Prime Minister Theresa May said student protesters miss out on important learning that can help them "develop into the top scientists, engineers and advocates that we need to help tackle this problem."
Many young people have been inspired by 16-year-old environmental activist Greta Thunberg, who has accused world leaders of not doing enough to prevent climate change.
In August last year, Greta began her campaign. She would be taken out of school by her dad on Friday (against her school's wishes) and she would then protest outside of Sweden's parliament building to encourage them to do something about the issue.
Her actions got her noticed and now her #FridaysForFuture have inspired other young people to take to the streets to ask governments to do more.
Everyone is welcome. Everyone is needed. Let's change history. And let's never stop for as long as it takes.
Greta - who is vegan, and refuses to fly in planes to help reduce her impact on the environment - has now been nominated for the Nobel Peace Prize.
This is a famous award given to people who have done a lot to try and help the world. Previous winners include Malala Yousafzai and Dr Martin Luther King.
But some people are not so happy with Greta's decision to skip school and think that the protests are a bad idea.
On Twitter, a user called Leah wrote: "Government has blocked our #ClimateStrike for tomorrow yet we wrote to them in time & followed all procedures."
"In Uganda one needs to be cleared by police before doing any public event of more than 20 people."
"I am sad but I won't give up. We'll do to it our way @GretaThunberg."
This is not the first time a protest like this has happened in the UK. Last month thousands of UK students took to the streets to protest climate change.
The National Association of Head Teachers did not support the strikes, saying "nothing is more important than a child's education".
Prime Minister Theresa May is also unhappy with the strikes.
Everybody wants young people to be engaged in the issues that affect them most so that we can build a brighter future for all of us... But it is important to emphasise that disruption increases teacher's workloads and wastes lesson time that teachers have carefully prepared for.