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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

The 18 minute mark is almost certainly a mistake. The source of this time limit is from Pesaḥim 46a

Mishna: [Regarding] ‘deaf’ dough, if there is [a dough] similar to it which has become leaven, it is forbidden. Gemara: What if there is no [dough] similar to it?―Said R. Abbahu in the name of R. Shimon b. Lakish: [The period for dough to become chametz is] as long as it takes a man to walk from Migdal Nunaiya to Tiberias, which is a mil. Then let him say a mil?―He informs us this, [viz.,] that the standard distance of a mil is as that from Migdal Nunaiya to Tiberias. R. Abbahu said in the name of R. Shimon b. Lakish: For kneading, for prayer, and for washing the hands, [the standard is] four mils.

And later in Pesaḥim 93b the Gemara states that on an average day one day's journey (from sunrise to sunset) is 40 Roman miles. This would mean that on that 'average' day of 12 hours sunlight the time it takes to walk one Roman mile would be 720 minutes divided by 40 Roman miles. We have thus reached our 18 minutes.. But there's this tiny teeny little problem... the actual distance from what was the area known as Migdal Nunaiya to Tiberias is closer to 6 miles. Manuscript copyists of the Talmud Bavli wouldn't know the actual distance from Migdal to Tiberias, so this is an error that is easy to overlook for long periods of time, even commentators who did not live in Israel nor knew archaeology might not catch it. Many would claim that this contradictory information isn't a problem and we should side with the Gemara and our commentators. Which is fine.... until you find out the Talmud Yerushalmi disagrees with the Talmud Bavli.

Mishna: [Regarding] ‘deaf’ dough, if there is [a dough] similar to it which has become leaven, it is forbidden. Gemara: What if there is no [dough] similar to it?―Said R. Abbahu in the name of R. Shimon b. Lakish: [The period for dough to become chametz is] as long as it takes a man to walk from Migdal Nunaiya to Tiberias, which is 4 mil.

So we have a disagreement to the power of 4, and this is not a reconcileable difference. But hey, at least we are closer to 6 miles. But there are many who would say that we don't hold the Yerushalmi as being authoritative...but then you have variant manuscripts of the Talmud Bavli which don't mention what the actual distance between Migdal to Tiberias is. These manuscripts include the Yemenite copies of Pesahim, and there are more. Unfortunately i don't have copies of these manuscripts on hand, but they would read like the following:

Mishna: [Regarding] ‘deaf’ dough, if there is [a dough] similar to it which has become leaven, it is forbidden. Gemara: What if there is no [dough] similar to it?―Said R. Abbahu in the name of R. Shimon b. Lakish: [The period for dough to become chametz is] as long as it takes a man to walk from Migdal Nunaiya to Tiberias.

If indeed the Talmud Bavli we have is in error, then the most reliable source we have is the Talmud Yerushalmi, which cites the distance as 4 mil. If this is the case, this would mean that you have approx 72 minutes rather than 18.

There are many divergent opinions regarding this topic. The Rambam in Halachot Chametz: Halacha 13 states:

As long as a person is busy with the dough, even for the entire day, it will not become chametz. If he lifts up his hand and allows the dough to rest so that [it rises to the extent that] a noise will resound when a person claps it with his hand, it has already become chametz and must be burned immediately. If a noise does not resound and the dough has lain at rest for the time it takes a man to walk a mil, it has become chametz and must be burned immediately.

The Rambam rules that as long as you are beating the dough, then even if you're beating it all day long, it will never become hametz. But he does agree with the 1 mil brought down by his manuscript of the Bavli, i don't know if he sources the Yerushalmi.

The RI'AZ is familiar with the statements of both Talmuds, and seems to try and reconcile them both by ignoring the Gemara and Rambam's statements that these times only apply to bread that is left sitting, according to his opinion:

Dough certainly becomes Hametz after 4 mil (approximately 72 minutes), even if one continues to work it.

For more information, including more precise citations of these sources, you can buy the newest volume of Hakirah [or you can wait a few months and the volume will become free], which is volume 18 and includes the essay: Chametz in Eighteen Minutes? An Inquiry into the Correct Text of the Talmud

http://www.hakirah.org/Volume%2018.htm

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Note that the Chakira article will be free in a few months anyway. –  Double AA Apr 28 at 23:40
    
Shiltei Hagiborim is saying that both figures are valid lahalacha. 1 mil if the dough is left alone for that much time, 4 mil if it's being worked over the whole time. –  Shamiach Apr 29 at 0:24
    
The Gemara discusses that the time of 1 mil or 4 mil is only for bread that has been left alone, if you are beating bread, then it can stand all day long. This is verified by the Rambam –  Aaron Apr 29 at 3:55
    
Halachot Chametz: Halacha 13 As long as a person is busy with the dough, even for the entire day, it will not become chametz. If he lifts up his hand and allows the dough to rest so that [it rises to the extent that] a noise will resound when a person claps it with his hand, it has already become chametz and must be burned immediately. If a noise does not resound and the dough has lain at rest for the time it takes a man to walk a mil, it has become chametz and must be burned immediately. –  Aaron Apr 29 at 3:55
    
Yes, and clearly Shiltei Hagiborim (or Riaz) is disagreeing with that. –  Shamiach Apr 29 at 18:49

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