system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do those yellow things form in the corner of your eyes when you sleep?
gpt
That's called rheum. Your eyes discharge a thin liquid, which normally you continually flush away during the day. This can combine with other gunk, like dead cells, dust, and so on, and helps keep your eye clean by flushing them out.

At night, you don't do much 'flushing' since you aren't blinking, and so the gunk can collect and dry.