לזכות הרב פנחס אלימלך בן ביילא פריידל לברכה והצלחה בכל מעשי ידיו
שֵׁשׁ מַעֲלוֹת בַּמִּקְוָאוֹת, זוֹ לְמַעְלָה מִזּוֹ, וְזוֹ לְמַעְלָה מִזּוֹ. מֵי גְבָאִים, שָׁתָה טָמֵא וְשָׁתָה טָהוֹר, טָמֵא. שָׁתָה טָמֵא וּמִלֵּא בִכְלִי טָהוֹר, טָמֵא. שָׁתָה טָמֵא וְנָפַל כִּכָּר שֶׁל תְּרוּמָה, אִם הֵדִיחַ, טָמֵא. וְאִם לֹא הֵדִיחַ, טָהוֹר:
Mikvaos 1-1:
שֵׁשׁ מַעֲלוֹת בַּמִּקְוָאוֹת, זוֹ לְמַעְלָה מִזּוֹ, וְזוֹ לְמַעְלָה מִזּוֹ. מֵי גְבָאִים, שָׁתָה טָמֵא וְשָׁתָה טָהוֹר, טָמֵא. שָׁתָה טָמֵא וּמִלֵּא בִכְלִי טָהוֹר, טָמֵא. שָׁתָה טָמֵא וְנָפַל כִּכָּר שֶׁל תְּרוּמָה, אִם הֵדִיחַ, טָמֵא. וְאִם לֹא הֵדִיחַ, טָהוֹר:
There are six degrees of mikvaos, each superior to the other. The water of pools [smaller than 40 seah] if an unclean person drank of it and then a clean person drank of it, he becomes unclean; If an unclean person drank of it and then drew water from it in a clean vessel, [the vessel] becomes unclean; If an unclean person drank of it and then a loaf of terumah fell in: If he washed [his hands] in it, it becomes unclean; But if he did not wash [his hands] in it, it continues to be clean.
if an unclean person drank of it and then a clean person drank of it, he becomes unclean - The Rambam learned that when the Tamei person drinks from the מי גאים, the water IN THE GROUND becomes Tamei which in turn is מטמא the טהור person. But most Meforshim understood that the water only becomes Tamei when it is removed from the pool and comes into contact with the Tamei person and then when it falls back into the pool it reamins Tamei and then those Tamei drops are מטמא the Tahor person.
Asked lots of people: Why aren't the טמא drops that fell back in בטלים in the רוב pure waters which remained pure due to the fact that they were attached to the ground?? So when the Tahor person then drinks, it will already have been purified??!
Rav Moshe Rozen [he was the Chavrusa of the Chazon Ish and the Rov of his town in Lithuania and then later moved to Brooklyn] answered that since one drop of water is מטמא a lot of water if the water is detached from the ground, we see that the מבטל can be like the בטל. The only reason the water in the ground is not טמא is because it happens to be in the ground. But the water inherently could become טמא if it was detached. In such a case we don't apply the laws of ביטול when טמא water falls in. [If I understood correctly, Rav Rozen was מכוון to the answer of the רי"ד in his Tshuvos 1-3 quoted by the Nosson Piryo].
