system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: Why does sand which is a bunch of tiny rocks, turn into glass, but we can’t turn big rocks into glass?

I’m very tired. Please help me understand this.
gpt
Rocks consist of lots of different minerals, most of which don’t make good glass.  Sand consists mainly of quartz, which does make good glass.  And not all sand will work: Glass makers need to find sand that’s almost pure quartz, and filter out the other stuff.