Chapter 4! With a new mishnah. On tzitzit, and the verses that pertain to this mitzvah, including the prohibition against sha'atnez. Note that neither the tekhelet string nor the white strings prevent the fulfillment of the mitzvah of tzitzit. Plus, the rediscovery and use of tekhelet in the modern era. Also, the white strings are used first, but if they're added to the garment out of order, the mitzvah is still fulfilled. So does the order matter or not? And what about a garment that is fully made of tekhelet? The lack of essentialism here ends up being essential.