system
Given the following post, respond as if you're commenting on Reddit:
human
eli5.. if light has no mass, why can't it escape from the gravity of a black hole?
gpt
Ok so General Relativity says space-time is curved. So let's imagine that we're dealing with a specific chunk of space-time, represented by a trampoline. Now let's put a lead ball in the center and say that represents our Sun. That's creating a distortion in our chunk of space-time, so if you put any object with mass near enough; it's going to roll/slide down that distortion in space-time right? And that's essentially gravity.    
But photons have no mass right? Yes, BUT they still follow the geometry of curved space-time. So as light passes by our lead ball in the center, it follows that distortion in space-time and so its path of travel is affected.   
Now with a black hole the trampoline example doesn't work so well, because space-time is 4 dimensional and a trampoline is not. But if you can imagine a lead ball so dense that it causes space-time to distort so much that it folds over on itself, now any light that enters the black hole can only travel around in the black hole (think of it like a loop, although it's not really that simple), but it cannot escape because the curvature of this chunk of space-time created by the distortion caused by the singularity is so great.