system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Where did all the money go during the great depression?

I've always wondered...
gpt
If you have a rock that other people are willing to pay $1000 for (part of a pet rock craze). You have $1000 worth of rock as part of your net worth. 

One day people are only willing to pay you $5 for that rock (pet rock mania over).

Your net worth is decreased by $995, even though nothing physical have changed.