system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Today, China's richest man lost $15 billion since his stocks crashed. Where'd that money go?

Where dat penny?
gpt
It doesn't have to *go* anywhere.

Imagine if you had a painting that you purchased for $13 billion—it's a very nice painting.  Other people agree that your painting is very very nice.  In fact, many people think that you got a good deal by buying it for "only" $13 billion and they make offers to buy it for $15 billion, but you're happy where you are—you want to keep the painting.

At this point you have $15 billion worth of painting.  There aren't $15 billion sitting around with your name on it, but the value of your assets includes that painting which most people agree is worth a lot.

Then one day people have a shift of opinion and decide that paintings aren't worth that much.  Suddenly nobody wants to give you anywhere close to $15 billion for the painting.  Perhaps the best offer is now $1 million—1/15,000 what it had been worth.  You still have the same assets, but now your assets aren't worth as much.

Stocks work essentially the same way.  You decide that you'd rather own stocks than money, but you still get to count the stocks as part of your net worth because you own them and they're worth money.  Then one day people decide that they don't want those stocks very much and the value of the stocks crash without you ever losing any asset and without any money changing hands.  The value of the stocks is just gone.