system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does applying toothpaste to a CD to help fix scratches work?

Is it just a hoax? I've tried before and never could get it to work, but my fallout 3 disc is scratched all to hell and I want to play it before 4 comes out. (I know 4 comes with a copy of 3 but I don't want 4 sitting on the shelf for a month before I can play it)
gpt
The data on a CD, DVD, etc is stored on the label side, on the surface of a thin sheet of metal that can be seen through the plastic. The top of a CD is usually pretty well protected by being a nice hard surface, but the bottom is just a plastic that is prone to being scratched.

If you have scratches on this side of the CD then it can deflect the laser that's trying to read the information. Polish the scratch off of the surface and the data can be read.

Toothpaste just happens to be a pretty good mild abrasive. Off the shelf CD repair kits work similarly and may have a clear compound to fill the scratches.