Toharot, Chapter Three, Mishnah Eight
1) A child was found next to dough with a piece of dough in his hand:
a) Rabbi Meir says that the dough is clean;
b) But the sages say that it is unclean, since it is the nature of a child to slap dough.
2) 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.
3) And in the case of a cow or a dog, if there was distance enough for it to lick its tongue.
4) And in the case of all other beasts, if there was distance enough for their tongue to dry.
5) 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.
Section one: A child can be assumed to have had contact with unclean things, such as a sheretz (a creepy crawly thing) or something else that was unclean (children play with yucky stuff). If a child is found playing next to some dough and he has some dough in his hands, we would think that it is nearly certain that the dough came from there. Nevertheless, Rabbi Meir rules lenientlyhe says that it is possible that a pure person gave the dough to the kid.
The other sages are not so lenient. They say that even if the child is not holding on to the dough, it is impure because children like to play with dough. Sort of like ancient play-doh!
Section two: The rest of the mishnah deals with dough that bears traces of an animal having pecked or licked it. Near the dough there is some liquid. We need to now figure out how likely it is that the animal licked the liquid and then brought it over to the dough before the dough dried. If the marking was made by a hen, then the dough is pure if the liquid is far enough away such that the hen could have dried its mouth on the ground. Evidently, hens do this. Who knew?
Section three: If the mark was made by a dog or cow, then they must have had enough time for their tongue to dry out. Dogs and cows lick their lipsthis I knew.
Section four: If it was made by any other animal, there must have been enough time for the liquid to dry off their mouths or tongues. In all of these cases, if the liquid can be assumed to have dried, the dough is pure.
Section five: Rabbi Eliezer ben Jacob appreciates that dogs are not stupid. Since there are water and food in the area, the dog would not be foolish enough to go for the water and not the food. Therefore the dough remains pure.