(Exodus 12:15) "Seven days shall you eat matzoth": I might think that all types of matzoth are understood (i.e., that all grains are permitted as matzoth). It is, therefore, written (Devarim 16:3) "Do not eat chametz upon it; (seven days shall you eat matzoth upon it"). Scripture speaks only of something that can become chametz (i.e., that can turn sour) — wheat, barley, rye, oat, and spelt. This excludes rice, millet, poppyseed, pulse, and sesame, which do not become chametz, but which putrefy.