One of my favorite facts gleaned from listening to Science & Futurism with Isaac Arthur is that black holes are almost banal in their simplicity particularly with regards to what an event horizon is. What goes up must come down unless it's traveling fast enough. Throw a ball at twelve thousand meters per second on Earth's surface (pretend atmospheric drag doesn't exist) and it is likely to end up traveling to the moon or beyond. Different spheres have different surface gravity and different escape velocities. Our Sun's is much higher than Earth's and bigger stars are higher still. The Milky Way Galaxy has various escape velocities from different points within it. So what does this elementary consequence of the laws of motion have to do with black holes?
The universe has a maximum speed at which causality propagates. For whatever reason this is also the exact speed that electromagnetic waves travel in a vacuum. There is a minimum speed which something must be traveling in order to continue traveling away from a gravitational field. Therefore, it stands to reason that an intense enough gravitational field would create a region of space that nothing escapes from. That region is the inside of an event horizon.
I've spent years hearing about how black holes break physics because blah blah general relativity blah blah quantum mechanics. I'm sure all of that is true but I wish it hadn't taken a decade for somebody to outright say that event horizons are just the point where the escape velocity exceeds the speed of light because that makes black holes about ninety percent less mysterious to me personally. Sorry to anybody for whom this was completely obvious from the first second that they had black holes explained to them but I spent a long time in the dark about what is actually a very intuitive phenomena that follows easily from basic mechanics and a superficial understanding of gravitational lensing.
IRON NODER XVI: MORE STUBBORN-HARD THAN HAMMER'D IRON