system
Given the following post, respond as if you're commenting on Reddit:
human
Why does an accretion disc fall into a black hole instead of orbiting like saturn's rings?

Is it something go do with Saturn's moons keeping the rings stable? Or maybe because the discs are made of mostly gas rather than bits of rock and ice?
gpt
Imagine two rings of dust or gas, orbiting completely circulars round a black hole, and very close to each other. Because the gravity of the black hole is so strong, the rings will be orbiting at very high speeds, but the inner ring significantly faster. If any particles (solid, liquid or gas) in the two separate rings come too close, they will experience friction. They will both heat up and the one closer to the black hole slows down as it is faster. Suddenly the closer particle is going too slow to orbit and falls in, whereas the outer particle is going to fast and flies out into another particle, causing more friction. Now imagine thousands upon thousands of tiny rings, which basically is a giant accretion disk, formed from a ripped up star or gas cloud. This disk is constantly creating friction with itself and so most of it falls in. A few particles of it however gain speed from it and fly out, though these are marginally small, and are normally pulled back in. The friction also heats up the disk to make it glow about as bright as a star.

Now the difference with Saturn is that it has much less gravity, so inner parts of the rings don't have to orbit much faster than the outer bits, thus friction is kept to a minimum and the disks are stable. 

**TL;DR: Friction**