Mishnayos Taharos Perek 3 Mishnah 8

A child was found next to dough with a piece of dough in his hand: Rabbi Meir says that the dough is clean; But the sages say that it is unclean, since it is the nature of a child to slap dough. Dough that bears traces of hens’ pickings and there is unclean liquid in the same house: if there was distance enough between the liquid and the loaves for the hens to dry their mouths on the ground, the dough is clean. And in the case of a cow or a dog, if there was distance enough for it to lick its tongue. And in the case of all other beasts, if there was distance enough for their tongue to dry. Rabbi Eliezer ben Jacob holds the dough to be clean in the case of a dog who is smart; for it is not its habit to leave food and go after the water.