Mishnah Mikvaos 6:6: If a jagged-edged [impure] earthenware vessel is in a mikveh [with its edges above the water's surface], and one immersed vessels in it, they are purified from their impurity [which they had before they were immersed], but they become impure due to [the protruding edges of] the earthenware vessel [which are impure and, as the immersed vessels emerge from the water but are still inside the space of the earthenware, render the water on them impure, which in turn renders those vessels impure]. If [however] any amount of water was floating above [the jagged edges], they are pure. A spring that flows [through and] out of an [impure] oven, of one descends and immerses in it, he is pure, but his hands are impure [since one's hands, but not one's body, are rendered impure by impure liquids, such as the liquid on his body once he emerges into the space of the impure oven]. If they [the spring's waters] rose above it [the oven] the height of his hands, even his hands are pure.

גִּסְטְרָא שֶׁבַּמִּקְוֶה וְהִטְבִּיל בָּהּ אֶת הַכֵּלִים, טָהֲרוּ מִטֻּמְאָתָן, אֲבָל טְמֵאִים עַל גַּב כְּלִי חֶרֶס. אִם הָיוּ הַמַּיִם צָפִים עַל גַּבָּיו כָּל שֶׁהֵן, טְהוֹרִין. מַעְיָן הַיּוֹצֵא מִן הַתַּנּוּר וְיָרַד וְטָבַל בְּתוֹכוֹ, הוּא טָהוֹר וְיָדָיו טְמֵאוֹת. וְאִם הָיוּ עַל גַּבָּיו רוּם יָדָיו, אַף יָדָיו טְהוֹרוֹת:

Mishnah Mikvaos 6:7: Mikvaot are [considered] mixed [when they are connected by an opening] the size of the tube of a flask, alike to it in its width, and alike to it in [the length of] its hollow, such that two fingers can be rotated in place [i.e. its diameter should be the length of two fingers]. If there is an uncertainty as to whether it [the opening] was the size of the tube of a flask or was not the size of the tube of a flask, it is invalid because it is [a requirement] from the Torah [to immerse in a valid mikveh, and therefore we rule stringently on the matter]. Similarly [we rule stringently in a situation of uncertainty] regarding an olive's bulk of a corpse, and an olive's bulk of a carcass, and a lentil's bulk of a vermin. Anything that will stand in [an opening] the size of the tube of a flask, reduces it [from being that size, and thus from being able to be a connection between mikvaot]. Rabbi Shimon ben Gamliel says: any thing that is created out of the water is pure [i.e. does not reduce the size of the opening].

עֵרוּב מִקְוָאוֹת, כִּשְׁפוֹפֶרֶת הַנּוֹד, כְּעָבְיָהּ וְכַחֲלָלָהּ, כִּשְׁתֵּי אֶצְבָּעוֹת חוֹזְרוֹת לִמְקוֹמָן. סָפֵק כִּשְׁפוֹפֶרֶת הַנּוֹד, סָפֵק שֶׁאֵינָהּ כִּשְׁפוֹפֶרֶת הַנּוֹד, פְּסוּלָה, מִפְּנֵי שֶׁהִיא מִן הַתּוֹרָה. וְכֵן כַּזַּיִת מִן הַמֵּת וְכַזַּיִת מִן הַנְּבֵלָה וְכָעֲדָשָׁה מִן הַשֶּׁרֶץ. כֹּל שֶׁיַּעֲמֹד כִּשְׁפוֹפֶרֶת הַנּוֹד, מְמַעֲטָהּ. רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל אוֹמֵר, כֹּל שֶׁהוּא מִבְּרִיַּת הַמַּיִם, טָהוֹר: