Keilim 9:3-4

Keilim 9:3

If a sheretz (vermin) was found under the floor of an oven, the oven remains ritually clean because one could say that it was alive while it was in the oven and died upon removal. If a (ritually unclean) needle or ring was found under the floor of an oven, the oven remains ritually clean because one could say that it was there before the oven was built over it. If the place where the needle or ring was found had ashes in it, the oven is rendered unclean because there’s no alternative hypothesis.

Keilim 9:4

Imagine that a sponge absorbed a ritually unclean liquid but is dry on the outside. If this sponge fell into the airspace of an oven, it is rendered unclean because eventually some of the unclean liquid will come out. The same is true of a piece of turnip or a reed that absorbed an unclean liquid, though Rabbi Shimon rules the latter two cases ritually clean.