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If flour (of any one of the five grains) is mixed with water, rapidly kneaded, and immediately put into an oven, the dough does not have time to rise. This is matza, the food we are commanded to eat at the Passover Seder, to remember the Exodus from Egypt and the haste with which the Jews left unable to wait for the dough to rise and so they made matza. As is written about the Israelites, “They baked unleavened cakes of the dough that they had taken out of Egypt, for it was not leavened, since they had been driven out of Egypt and could not delay” (Exodus 12:39).
The verse describes matza as unleavened bread, which implies it is a type of bread, and thus, like bread, must be made from one of the five grains. Although rice is similar to the five types of grain and it does rise, it does not undergo a complete fermentation process. Therefore, the prohibitions against ĥametz on Passover do not apply to rice, and matza made of rice cannot be used to fulfill the mitzva at the Seder.
As long as dough is in motion being kneaded, it does not become ĥametz. However, once it sits motionless for eighteen minutes, the leavening process begins, and all the ĥametz prohibitions apply to it. In a hot environment, the leavening process is accelerated, and the dough may become ĥametz in even less than eighteen minutes. Cracks appearing in the dough are a physical indication it has become ĥametz, even if it has been less than eighteen minutes since the dough was kneaded; apparently either the bakery or the dough was hot enough to accelerate the leavening process.
