Water to be used in matzah baking must be left to stand overnight (to ensure that it is allowed to cool). This water is then referred to as “mayim shelanu” (water that has “slept”).
As mayim shelanu is necessary for the preparation of each day’s production, the mashgichim supervising a matzah bakery must take care that:
the water tanks are filled immediately prior to nightfall at the Ein chinya - a small spring near Jerusalem
there is sufficient mayim shelanu for the next day’s matzah production;
no additional water is added to the mayim shelanu tank during the day’s production