In history today, we learned part of what caused the Great Depression. People thought that when they bought things with credit, that it was free. They didn't realize that you had to pay the same amount plus extra. So when they were buying a bunch of 'free' things, and then had to pay for them, they didn't have the money available. So they went to their banks to get some money out of some stock that they had but the banks didn't have the money. It was crazy! :)