Money

That last post got me thinking. What is money? I know it's a very philosophical question but I had to ask someone. It's just a fancy. Human imagination. A figment of our thoughts. That's it. If some aliens landed on our planet who didn't have money back home, what'd they do? They'd probably throw all of the supposed 'printed paper' in the bank down the drain. Then how come half the world is working and living and dying for paper? Forget aliens. Suppose that all of our own people, humans, Homo sapiens just lost their minds some day. A mass brain destruction or something. The whole world. Everyone. What value will money have then? What will be the use of those bits of paper and those round pieces of metal? Nothing. Nil. Zero. I just don't understand. I know that I'm still young, and my mind and outlook isn't as developed as an adult's, but this is one question I'd definitely like answered.