We might not remember much from high school science, but we do know that oxygen and fuel = fire.
This is the process of combustion, which brings together oxygen and a fuel source.
So when we look at the big ball of flames in the sky - the Sun - what's actually going on there?
Basic science tells us there's no oxygen in space - that's why astronauts have to get kitted out in heavy suits and helmets, so they can breathe out there - so how could the Sun be on fire?
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Well, one scientist has the answer.
"In reality, the Sun is not burning, because the Sun is not fire," said Dr Ryan French in a TikTok video.
French is a solar physicist and author, and on his TikTok page - which has 49.7k followers - he says his job is dedicated to researching the Sun, meaning he's well placed to give us the low-down.
"The Sun is not powered by combustion, but instead a different energy source called nuclear fusion, taking place in the core of our Sun and every other star as well," French explained.
He said the whole lifetime of any star is a "constant battle" between the "gravity of the mass of the star pushing it inwards, and a resistive force pushing outwards".
For most stars, this outward force is the process of nuclear fusion, where hydrogen atoms are squished together in an environment of huge pressure and a high temperature. This creates a helium atom, which French said is lighter than hydrogen - so all this excess mass is released as energy.
This is a huge amount of energy being released from the core of the Sun - and this is transported to the surface "via radiation, convection and conduction", French said - making it look like the Sun is on fire.
And safe to say people are loving this new info in the comments section.
"I am literally shocked that the sun is not on fire. Super interesting!!" one TikToker wrote.
Another agreed: "Very educational and well put, good video."
And others were keen to learn more about the Sun from a specialist.
A TikToker asked: "Have you seen the solar activity comparison from 2019 to now? If so, if that normal for the Sun to fluctuate that much?"
To which French replied with his expert answer: "Sun follows an 11-year cycle of increasing/decreasing activity!"