What did Newton understand that was such a revolution? Can someone just... explain gravity to me? Let's start with a ...

Besides quantum theory, general relativity is one of two pillars of modern physics – our working theory of gravity and of the very large, of planets, galaxies and the universe as a whole.

But how does gravity work in the first place? What did Newton understand that was such a revolution? Let's start with a definition. Gravity, or gravitational attraction, is the tendency of mass to ...