Kelim, Chapter Eight, Mishnah Six
1) A leavening pot with a tightly fitting lid which was put in an oven, and there was some leaven and a sheretz within the pot, but there was a partition (of inedible bread) between them, the oven is unclean but the leaven is clean.
2) But if it was an olive’s bulk of corpse, both the oven and the house are unclean, and the leaven remains clean.
3) If in the partition there was an opening of one handbreadth, all become unclean.
Section one: The mishnah describes a situation where a special pot used to leaven bread has a tight lid and was placed in an oven. Inside the pot there are two parts separated by a piece of inedible bread. On one side of the pot is some leaven and on the other side is a sheretz. The sheretz does not defile the leaven on the other side of the pot because it is totally separated, from the lid to the bottom. However, the lid does not stop the impurity from leaving the pot through its top. Therefore, the sheretz’s impurity defiles the oven.
Section two: Similarly, if there is an olive’s worth of corpse in the pot, its impurity escapes and defiles the entire house.
Section three: If there is a hole in the partition at least one handbreadth wide, then the partition no longer separates the sheretz from the leaven, and even the leaven is impure.