system
Given the following post, respond as if you're commenting on Reddit:
human
How do/can black holes "grow"?

Perhaps this has been answered here or elsewhere, but I can't seem to find a clear answer. 

Maybe this is a dumb question, and maybe I've already read the answer before and forgotten, but: How is it that black holes can "grow" by consuming matter/mass? If matter beyond the event horizon is "gone" (though, I guess Hawking radiation would suggest otherwise?) why should the ingestion of matter after creation of the singularity cause the black hole to "grow" or otherwise become more dense? It makes sense to me that the aggregation of massive objects would do so, but does a singularity have mass, in the traditional sense? I guess what I'm struggling with is the idea that such mundane rules could apply to something so seemingly peculiar - is it just that simple, or is the actual phenomenon also appropriately exotic?
gpt
Are you asking about a mechanism?

Because if so, they just *do,* by conservation of mass/energy.

When matter (or energy, such as photons) cross the event horizon it is absolutely doomed to end up at the singularity. That's just the way spacetime works in the black hole. Basically, geodesics (which are orbital trajectories in general relativity) terminate at the singularity. It means that stuff just piles up at that point. By conservation of mass/energy, the mass/energy of the particle that went in is now a part of the singularity, and thus the black hole.