What is Money?

I read a terrific book: Money: The True Story of a Made-Up Thing, by Jacob Goldstein last year. I spent 35 years in the investment business, and the thing people wanted to know about money in that realm was: How Big of a Pile of Money Can Be Accumulated?
But the concept of Money itself is very interesting. Why do people trust that money has value? The bills pictured below were once accepted as real money; you could buy dinner with them. How does that relate to Cryptocurrencies like Bitcoin? I still carry around a real Zimbabwe 100 Trillion Dollar Bill. It's worth about $1 as a collectible, you can buy them on Ebay.
Going down that road led me to write: The Bitcoin Gambit.