Flat-Earthers have a lot of explaining to do. For starters, they have to explain why we have photos of the Earth taken from space.

Then they have to explain why not one human being in the history of the planet has ever found the ends of the Earth despite several attempts, including one couple who recently attempted to reach the edge, thought to be somewhere near Sicily.

On a long list of things that flat-Earthers have to account for, they also have to explain why, if the Earth is so flat, all the other planets appear to be big old balls floating around the Sun. It is this climate activist Greta Thunberg took a shot at when posting a tweet about her 18th birthday this week.

“Thank you so much for all the well-wishes on my 18th birthday!" she wrote on Twitter. “Tonight you will find me down at the local pub exposing all the dark secrets behind the climate- and school strike conspiracy and my evil handlers who can no longer control me! I am free at last!!”

Thunberg, who is well versed in responding to her critics and trolls, doesn't just address accusations her climate activism is a result of manipulation by adults due to her child status before now, or that the climate crisis itself is a conspiracy theory. On her top are the words "Flat Mars Society", mocking the strange belief that just one planet alone – take your pick – is flat.

Bizarrely, flat-Earthers have already responded to the idea several years ago, when Elon Musk tweeted about "flat Mars," asking "why is there no Flat Mars Society?"

"Hi Elon, thanks for the question," the Flat-Earth Society replied on Twitter, quite politely. "Unlike the Earth, Mars has been observed to be round. We hope you have a fantastic day!"

Environmentalist Thunberg first started her "School Strike for Climate" movement in 2018, inspiring millions of children to take part.

Following a gap year in which she sailed across the Atlantic and attended the UN Climate Action Summit, Thunberg returned to school and activism, though she has had to move this activity online due to the ongoing COVID-19 pandemic.