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Are there sources EXPLAINING why we have to make matzah within 18 minutes? I understand having a time limit if in fact the reason we eat matzah is to reenact the speed which we left Egypt (that explination is questionable as we ate matzah in Mitzraim anyway, but that is food for another question) But why 18 minutes, it seems a bit like an arbitrary number.

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The short answer is because we are concerned about its being chametz if it bakes in more than 18 minutes. That is, the eighteen-minute limit has nothing to do with the rules of matza and is because of the rules of chametz. But that of course leads to another question: where does the eighteen-minute limit in the rules of chametz come from? –  msh210 Jan 2 '12 at 21:53

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The Gemara in Pesachim 46a (brought in the Rambam Laws of Chametz 5:13 and the Shulchan Aruch OC 459:2) says that if one leaves dough for the length of time that it takes to walk 1 "mil" (a Talmudic unit of distance) then it becomes Chametz. Opinions regarding the time it takes to walk a mil vary from 18-24 minutes, and here we are strict to take the shorter time.

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but why is that the law about chametz? where does that idea come from? –  morah hochman Jan 2 '12 at 23:45
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@morahhochman Are you asking how Chazal knew that it takes 18 minutes for dough to rise? The answer is probably the same way they knew it takes 13.5 minutes for three stars to come out, and that eating one date is enough to remove a feeling of suffering on Yom Kippur. They just did. It's like this with all shiurim. Chazal had to define some boundary to differentiate between two statuses. –  Double AA Jan 2 '12 at 23:49
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@morahhochman, just to add to DoubleAA's point, further on (48b) the Mishnah defines the first stage of dough rising as "when its surface pales, like a person whose hair is standing on end." So indeed Chazal may simply have found by observation that under normal conditions it takes a 1-mil walk for dough to reach this stage. –  Alex Jan 3 '12 at 3:36

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