Mishnah Taharos 8:7: The outsides of vessels which were rendered impure by liquids, Rabbi Eliezer says: they can render liquids impure but do not invalidate [terumah] foods [by rendering htem impure]. Rabbi Yehoshua says: they can make liquids impure and invalidate [terumah] foods. Shimon the brother of Azaryah says: neither this or that, but rather liquids that became impure from the outsides of vessels render impure one [item upon contact, rendering it a second degree level of impurity], and [that item, in turn, is able to] invalidate one [more item, rendering it a third degree level of impurity, which invalidates terumah foods]. Behold it [is as though the terumah food] says [to the liquids], "that which renders you impure [i.e. the outsides of vessels] does not impurify me, yet you have rendered me impure!"

אֲחוֹרֵי כֵלִים שֶׁנִּטְמְאוּ בְמַשְׁקִים, רַבִּי אֱלִיעֶזֶר אוֹמֵר, מְטַמְּאִין אֶת הַמַּשְׁקִין וְאֵין פּוֹסְלִין אֶת הָאֳכָלִין. רַבִּי יְהוֹשֻׁעַ אוֹמֵר, מְטַמְּאִין אֶת הַמַּשְׁקִין וּפוֹסְלִין אֶת הָאֳכָלִין. שִׁמְעוֹן אֲחִי עֲזַרְיָה אוֹמֵר, לֹא כָךְ וְלֹא כָךְ, אֶלָּא מַשְׁקִין שֶׁנִּטְמְאוּ בַאֲחוֹרֵי הַכֵּלִים, מְטַמְּאִין אֶחָד וּפוֹסְלִין אֶחָד. הֲרֵי זֶה אוֹמֵר, מְטַמְּאֶיךָ לֹא טִמְּאוּנִי, וְאַתָּה טִמֵּאתָנִי:

Mishnah Taharos 8:8: An inclined kneading trough with dough above and liquid dripping [enough to wet something that touches it] below, if there are three pieces [of impure dough] that [together] make up the equivalent to [the volume of] an egg, they do not combine [to constitute a sufficient measure of foods to render the liquid impure; this is because the incline of the container prevents us from considering the liquid to be touching all three pieces at once]; but two [pieces of dough that together add up to the volume of an egg] do combine [to constitute a sufficient measure to render the liquid impure]. Rabbi Yose says: even two do not combine, unless they were pushing against the liquid [preventing it from falling down the incline]. And if the liquid was standing [i.e. not on an incline], even [if the egg's volume of dough was crumbled into pieces] the size of mustard seeds, they do combine [to constitute the sufficient measure to render the liquid impure, since the standing liquid is considered to be touching all the pieces at once]. Rabbi Dosa says: crumbled food does not combine.

עֲרֵבָה שֶׁהִיא קְטַפְרֵס, וְהַבָּצֵק מִלְּמַעְלָן וּמַשְׁקֶה טוֹפֵחַ מִלְּמַטָּן, שָׁלשׁ חֲתִיכוֹת בְּכַבֵּיצָה, אֵינָן מִצְטָרְפוֹת. וּשְׁתַּיִם, מִצְטָרְפוֹת. רַבִּי יוֹסֵי אוֹמֵר, אַף שְׁתַּיִם אֵינָן מִצְטָרְפוֹת, אֶלָּא אִם כֵּן הָיוּ רוֹצְצוֹת מַשְׁקֶה. וְאִם הָיָה מַשְׁקֶה עוֹמֵד, אֲפִלּוּ כְעֵין הַחַרְדָּל, מִצְטָרֵף. רַבִּי דוֹסָא אוֹמֵר, אֹכֶל פָּרוּד אֵינוֹ מִצְטָרֵף: