Kelim, Chapter Nine, Mishnah Four
1) A sponge which had absorbed unclean liquids and its outer surface became dry and it fell into the air-space of an oven, the oven is unclean, for the liquid would eventually come out.
2) And the same with regard to a piece of turnip or reed grass.
a) Rabbi Shimon says: the oven is clean in both these cases.
Section one: Since the unclean liquids in the sponge will eventually seep out, the oven is unclean as soon as the sponge enters. In other words, since the liquids will eventually seep out, we look at them as if they are already out, and as we have learned, unclean liquids can defile an oven.
Section two: Evidently, turnips and reed grass were also used in a “sponge-like” manner, to gather in liquids and then to squeeze them out later. Therefore, according to the first opinion, if a piece of turnip or reed grass that has been used to soak up unclean liquids falls into the oven, the oven is immediately unclean, the same as it is with a sponge.
Rabbi Shimon says that in these two cases the liquid will not seep out like it does from a sponge and therefore the oven remains clean.