What every teenager of 2020's knows but minds of ancient Greece did not?

If you’re aiming for “as far as we know, not a single Greek suggested” things like that the earth revolves around the sun” or “everything is made of atoms” do not qualify, and even “that atoms can be split” wouldn’t (https://en.wikipedia.org/wiki/Atomism proposes “that the physical universe is composed of fundamental indivisible components known as atoms”, but the Greeks did not make the mistake of calling what we call atoms atoms)

Chemistry is rather new, though. I don’t think any Greek remotely knew that water is H₂O, and also guess the majority of teenagers would know.

The existence of more than 5 planets also is a likely candidate. The Greeks didn’t know that, and knowledge of that in the English-speaking world under teenagers is helped by the fact that one of them is called Uranus.

That light comes from the light source and is reflected into our eyes, allowing us to see.

Not that our eyes are the light source, as Plato wrote in the ~300BCs.

That chocolate, potatoes, avocado, corn and tomato are things. Along with the entirety of the America's.

That actual democracy can work?

The Sun–Earth distance is 93 million miles. The ancient Greeks could not even estimate this value.