The Halacha states (Shulhan Aruch 455) states that the water used to knead the Massa dough must be "Mayim SheLanu"-water that "rested" overnight. That is, it must be drawn at least one day prior to the baking and left overnight in a cool place. As heat is a prime cause of leavening, this measure insures that the water will release any heat absorbed over the course of the day. The custom is to draw the water in the late afternoon, before sundown, in order to accommodate a minority opinion that holds that underground water is coolest at the end of the day.
"Mayim SheLanu" is a mandatory aspect of baking Massot. Hacham Ben Sion rules that it is unacceptable to use other means of cooling the water, such as refrigeration or ice, in order to use water drawn on the same day as the baking. While traditionally, water for baking Massot was drawn from wells and rivers, he rules that it is permissible to use tap water from the faucet. He recommends storing the water in glass or ceramic vessels. Plastic is problematic because it tends to leave an odor or chemical residue in the water.
SUMMARY
1. The water used to knead the Massa dough must be "Mayim SheLanu"-water that was drawn the previous day.
2. It is permissible to use tap water to bake Massot.
3. It is preferable to store the water in glass or ceramic containers.
