Imagine you have a Spiderman Comic. Not even a really rare one. Just a regular old Spiderman comic. It's worth $1.

Then an interview surfaces where Stan Lee says that's his very favourite issue of Spiderman ever. Now it's worth $100.

Then you find out your copy is signed by Stan Lee himself. Now it's worth $1000.

Then it comes to light he signed ten copies of this one comic. A guy in Sweden owns the other nine. Now yours is worth $5000.

Then the guy in Sweden says that all of his copies burned in a fire. Now it's worth $10,000.

But then the guy is caught selling five copies of the comic. Turns out he made up the story to drive the price up. Yours is worth $5,000 again.

Then it turns out they weren't signed by hand, but with a stamp. Now it's worth $2,000. Then it turns out there's actually hundreds of them. Now its worth $500.

Then a story emerges that Stan Lee didn't draw any comics, or write anything, that it was all lies and self-promotion, and also he was secretly a communist and a spy for the Swedes. Every hates Stan Lee and all twenty-eight Spiderman movies currently in production are cancelled.

Now it's worth $1 again.

In all cases, your actual possessions haven't changed, but your net worth has been all over the place.

Expanded to a grand scale, that's basically how the stock market works.