The manifold precautions which must be taken at the various stages of the matzah's production are designed to prevent any fermentation whatsoever of the flour. The flour suitable for the baking of matzah can be divided into three categories of decreasing stringency: (1) "guarded flour," which is closely supervised from the time the wheat is harvested and is used for the preparation of matzah shemurah; (2) "Passover flour," where supervision to prevent fermentation begins with the milling of the wheat; (3) "ordinary flour," which does not have supervision until the point of being mixed and is used to make "ordinary," or machine, matzah. Under normal conditions of climate and temperature, flour mixed with water begins to ferment in approximately 18 minutes. Should the water be above room temperature, however, the process is accelerated, but it can be delayed by the continual manipulation of the dough. In order to prevent water from becoming too warm, only "mayim she-lanu" – "water which has rested" (Pes. 42a) – i.e., water which has been left in a vessel overnight to reach room temperature – is used in the baking of matzah, and thereafter the mixture of flour and water is constantly manipulated until it is ready for baking. Care must be taken that the whole process from kneading to final baking does not exceed the 18 minutes. No ingredients other than flour and water are permitted for Passover matzah. Although it is accepted by most decisors that salt is not a fermenting agent, its use in matzot is forbidden in order to prevent fermentation (Sh. Ar., OH 455:5). Some Yemenites, however, do have the custom of baking their Passover matzah using salt.