Mishnah Taharos 3:8: If a baby was found next to a pile of dough and a piece was in his hand [and it is unclear whether he himself took the piece off rendering all the dough impure, or if someone else tore the piece off for him], Rabbi Meir considers it pure. And the Sages consider it impure, because it is the way of babies to touch [the dough]. Dough which has peck-holes from chickens and there are impure liquids in the house, if there is enough space between the liquids and the loaves for them to wipe their beaks on the ground, then they are pure. And regarding a cow or a dog, enough space for them [to have time] to lick [their lips with] their tongues. And with all other animals, enough space [for them to travel] for it to be dried up. Rabbi Eliezer ben Yaakov considers it pure in the case of a dog, since he is intelligent, and it is not his way to leave the food and go get water [first, before he finishes eating; and therefore it can be assumed that he did not touch the loaves after touching the impure liquids].

תִּינוֹק שֶׁנִּמְצָא בְצַד הָעִסָּה וְהַבָּצֵק בְּיָדוֹ, רַבִּי מֵאִיר מְטַהֵר. וַחֲכָמִים מְטַמְּאִים, שֶׁדֶּרֶךְ הַתִּינוֹק לְטַפֵּחַ. בָּצֵק שֶׁיֶּשׁ בּוֹ נְקִירַת תַּרְנְגוֹלִים, וּמַשְׁקִין טְמֵאִין בְּתוֹךְ הַבַּיִת, אִם יֵשׁ בֵּין מַשְׁקִין לַכִּכָּרוֹת כְּדֵי שֶׁיְּנַגְּבוּ אֶת פִּיהֶם בָּאָרֶץ, הֲרֵי אֵלּוּ טְהוֹרִין. וּבְפָרָה וּבְכֶלֶב, כְּדֵי שֶׁיְּלַחֲכוּ אֶת לְשׁוֹנָם. וּשְׁאָר כָּל הַבְּהֵמָה, כְּדֵי שֶׁיִּתְנַגֵּב. רַבִּי אֱלִיעֶזֶר בֶּן יַעֲקֹב מְטַהֵר בְּכֶלֶב, שֶׁהוּא פִקֵּחַ, שֶׁאֵין דַּרְכּוֹ לְהַנִּיחַ אֶת הַמָּזוֹן וְלֵילֵךְ לַמָּיִם:

Mishnah Taharos 4:1: One who throws an impurity from place to place, [such as] a [pure] loaf into [a collection of impure] keys, or a [impure] key into [an collection of pure] loaves, they are assumed to be pure. Rabbi Yehuda says: a loaf into [impure] keys is impure, but a key into [pure] loaves, [they are] pure.

הַזּוֹרֵק טֻמְאָה מִמָּקוֹם לְמָקוֹם, כִּכָּר לְבֵין הַמַּפְתְּחוֹת, מַפְתֵּחַ לְבֵין הַכִּכָּרוֹת, טָהוֹר. רַבִּי יְהוּדָה אוֹמֵר, כִּכָּר לְבֵין הַמַּפְתְּחוֹת, טָמֵא. מַפְתֵּחַ לְבֵין הַכִּכָּרוֹת, טָהוֹר: