What was the Great Depression, and why did so many people lose their jobs?

Picture the world in the 1920s as a party that wouldn't stop. Money felt easy, jobs were everywhere, and people were sure tomorrow would be even richer. Then, almost overnight, the music stopped. The 1930s became known as the Great Depression — about a decade when much of the world's economy got stuck in a deep, gloomy slump. So what knocked the legs out from under the party?
First, you need one key idea: an economy is just people buying and selling from each other. Your spending becomes someone else's paycheck, and their spending becomes yours. When everybody is buying, the whole circle hums along happily. But this circle can spin the other way too — and that's the trouble.
