Sometimes developing detailed knowledge of an organism can unlock answers to broad biological questions. This talk will focus around green millet, Setaria viridis, a little weed that most people encounter in sidewalk cracks. Surprisingly, this unassuming plant has given us information about how plants are domesticated, how flowers are arranged on the plant, and how evolution shapes odd new structures using standard genetic tools. At the same time, it shows the importance of using tools from taxonomy, molecular sequence analysis, quantitative genetics, and genomics. A weedy green millet was domesticated thousands of years ago in China to produce foxtail millet, a source of grain. Using quantitative genetics we have identified genes that were selected during the domestication process. The two millets are part of a large group of 100 species that have diversified independently in Asia, Africa, and South America. Difficulty in identification of the species led us to develop a phylogeny and a monograph of the genus Setaria. Green millet has become a weed in North America (where it is not native), along with another Asian species and an African species. The two Asian ones may hybridize, but the African one does not. The genome sequence of green millet is available, and the plants can be transformed. We have preliminary data to show that the genetic toolkit is similar to that in maize and sorghum, but with interesting modifications. The millets can be recognized easily by distinctive sterile branches in the inflorescence. These are unknown in any other plant, and we know some of the genes that have to be modified to create them. Green millet thus tells us about how plants can be modified by natural and artificial selection.