Makhshirin, Chapter Six, Mishnah Three
1) All eggs are presumed to be clean except those of sellers of liquids.
a) But if they sold with them dry produce, they are clean.
2) All fish are presumed unclean.
3) Rabbi Judah says: pieces of aylatit and Egyptian fish which arrives in a basket, and Spanish tunny, these may be presumed clean.
4) All kinds of brine may be presumed unclean.
5) Concerning all these an am ha-aretz may be trusted when he declares them to be clean, except in the case of small fish, since they are usually stored with any am ha-aretz.
6) Rabbi Eliezer ben Jacob says: clean brine into which water fell in any quantity must be deemed unclean.
Section one: Eggs sold anywhere are presumed to be clean unless they are sold by sellers who also sell liquids. If the person also sells liquids we can assume that he touched the eggs with wet hands. This would have made them susceptible to impurity. Then when an impure person in the marketplace touched them, they would become defiled.
We should note again that defiled eggs can be eaten. The only real relevance would be for a person who only wants to eat pure food.
However, if the person who sold liquids sold the eggs near dry food we can assume he would dry his hands off first because he wouldn’t want the dry food to get wet. Therefore the eggs can be assumed not to have come into contact with liquid.
Section two: While alive, fish are not susceptible to impurity (this is true of all animals). However, when a fisher kills the fish there is still water on it. This water causes it to be susceptible to impurity. When impure people touch the fish, it becomes impure.
Section three: According to Rabbi Judah, these types of fish live long after they have been caught. By the time they die there will be no water left on them. Since they are not again moistened, they were not made susceptible to impurity.
Section four: The brine that comes out of fish is mixed with water. This makes it susceptible to impurity. Since we can assume that an impure person touched it, it is assumed to be impure.
Section five: In all of the cases discussed above, eggs, fish and brine, an am haaretz, a person who doesn’t generally observe the laws of purity, is believed to say that the food has not come into contact with water and therefore it has not become susceptible to impurity. The only exception is small fish which, according to the mishnah, are often left with another am haaretz. Since the am haaretz who says they are clean cannot know if the other am haaretz defiled them or got them wet, we can’t believe the first am haaretz.
Section six: If there is brine that we know to be clean because there is no water in it and then even the smallest amount of water falls in, that small amount of water makes the brine susceptible. We might have thought that since the brine is also liquid, the small amount of water is nullified. Rabbi Eliezer ben Yaakov teaches that it is not.