Mishnah Taharos 1:7: Pieces of dough which are touching each other or loaves that are touching one another, if one of them is rendered impure by [touching] a vermin, they are all [rendered] primary [in their impurity; i.e. they all become of a first degree of impurity]. If they were [then] separated, they are all [still] primary [in their impurity]. Regarding liquids [that are similarly touching each other, if one liquid is rendered impure by impure liquids, which are always of a first degree level of impurity, then] they are all of a second degree [level of impurity]; if they were [then] separated, they are all [still] of a second degree. If [they were made impure] by hands [which are of a second degree level of impurity], they are all of a third degree.

מִקְרָצוֹת נוֹשְׁכוֹת זוֹ בָזוֹ, וְכִכָּרִים נוֹשְׁכִין זֶה בָזֶה, נִטְמֵאת אַחַת מֵהֶן בְּשֶׁרֶץ, כֻּלָּן תְּחִלָּה. פֵּרְשׁוּ, כֻּלָּן תְּחִלָּה. בְּמַשְׁקִין, כֻּלָּן שְׁנִיּוֹת. פֵּרְשׁוּ, כֻּלָּן שְׁנִיּוֹת. בְּיָדַיִם, כֻּלָּן שְׁלִישִׁיּוֹת. פֵּרְשׁוּ, כֻּלָּן שְׁלִישִׁיּוֹת:

Mishnah Taharos 1:8: A piece of dough that was primary [i.e. of a first degree impurity], and one attached other pieces to it, they are all primary; if one [then] separated them, it [the original piece] is primary, and all the others are second degree. If it [the original piece] was second degree and one attached other pieces to it, they are all second degree; if one [then] separated them, it is second degree, and all the others are third degree. If it was third degree and one attached other pieces to it, it is third degree, and all the others are pure, whether they were separated or not.

מִקְרֶצֶת שֶׁהָיְתָה תְחִלָּה וְהִשִּׁיךְ לָהּ אֲחֵרוֹת, כֻּלָּן תְּחִלָּהּ. פֵּרְשׁוּ, הִיא תְחִלָּה וְכֻלָּן שְׁנִיּוֹת. הָיְתָה שְׁנִיָּה וְהִשִּׁיךְ לָהּ אֲחֵרוֹת, כֻּלָּן שְׁנִיּוֹת. פֵּרְשׁוּ, הִיא שְׁנִיָּה וְכֻלָּן שְׁלִישִׁיּוֹת. הָיְתָה שְׁלִישִׁית וְהִשִּׁיךְ לָהּ אֲחֵרוֹת, הִיא שְׁלִישִׁית וְכֻלָּן טְהוֹרוֹת, בֵּין שֶׁפֵּרְשׁוּ בֵּין שֶׁלֹּא פֵרְשׁוּ: