I have noticed a misunderstanding about borrowed money. I have noticed that sometimes students (and my own children!) think of borrowed money as their own money. Perhaps this is because they get to spend it, perhaps this is because of the proliferation of credit, but it is a weird thing to me. As I think about what mathematical concept it relates to, I am thinking of adding integers. This is a concept that I have seen Geometry students struggling with . . . and if you don't get what adding a negative number means then you might misunderstand what spending borrowed money means. Simple, but common.