system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we get "sand" in the corners of our eyes when we sleep? And why do we not get it during the day when we're awake?

Just wondering, can someone help me out?
gpt
The "sand" is actually dried tears.

That is, tears are similar to salt water -- liquid with salts in them. Overnight, when the tears at the edges of your eyes dry, the salt is left behind as "sand".