The modern Mormon understanding of tithing is rooted in D&C 119. According to Joseph Smith’s revelation, after the Saints contributed all of their surplus property to the bishop, they were to pay one-tenth of their “interest” annually.
And what does “interest” mean for tithing purposes? It turns out that, whatever we thought it was, we were wrong. We have new information that gives us a better idea of the original intent, and the kind folks over at Juvenile Instructor invited me to write about it over there. If you’re interested, check it out!