We should not be allowed to eat מצה on פסח! After all, it's made with מים שלנו ("our water"; that is, water that's been sitting out for hours and hours), and חז״ל say there's a risk of poisoning, ר״ל, from water that's been sitting out for so long. What gives?
closed as too localized by Isaac Moses♦ Mar 12 '12 at 5:14
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Okay here goes: normally issurim are batel b'shishim and venom is not. (See Magen Avraham with regards to fish-and-meat). But on Pesach, chametz isn't, so the halacha of Batel B'Shishim complained bitterly to Hashem "what about me!?" (his normally-bored kid brother, Batel B'rov, was having a grand old time playing with kitniyos galore, though he had to do so very quietly). So Hashem told Batel B'Shishim that it could spend Pesach dealing with snake venom.
The answer is that on ערב פסח we say "כל חמירא דאיכא ברשותי... ליבטל", which includes even the חמירא סכנתא מאיסורא; thus, there's no need for concern about the water.
This also explains why we don't eat the מצה until פסח. If it were allowed even ערב פסח, people would err and eat it before the ביטול, which is dangerous, so a לא פלוג was instituted to cover the whole day. (The לא פלוג was not extended into פסח itself, because if it were then we wouldn't be able to do יחץ, which is a פלוג.)
Another answer is that indeed the שלנו in מים שלנו is significant. Since it's "our water," then it's not subject to the law about poisoning, because אין אדם אוסר דבר שאינו שלו.
But, you'll object, a snake is not an אדם. The answer is that once it emits venom it has the status of one, as it says, אם נשך הנחש - את איש, "If the snake bites - [it is] a person" (Num. 21:9).
Actually, the only issur is by mayim megulin, water of exiled people. Water used to celebrate geulah is exempt from this prohibition.
In fact, it is in recognition of this freedom from our Egyptian masters and our newly found rights to ownership that we call this water "mayim shelanu".
The truth is we can not eat the Matza for a few reasons.
1 - It is called Lechem.
2 - It is Lechem Oni and Oni did not give us permission to eat his bread.
3 - The problem mentioned of Mayim Shelanu.
However we have a Klal "Ain Isur Chal Al Isur" therefore we are allowed to eat it.
In addition the Gematria of מים שלנו = נרצה עונה so Hashem forgives us for doing this.
The danger is with uncovered water, the water used for Matzos is left covered.