Kelim, Chapter Nine, Mishnah Seven
1) If there was netting placed over the mouth of an oven, forming a tightly fitting lid, and a split appeared between the oven and the colander, the minimum size [to allow impurity to enter] is that of the circumference of the tip of an ox goad that cannot actually enter it.
a) Rabbi Judah says: it must be one into which the tip can actually enter.
2) If a split appeared in the netting, the minimum size is the circumference of the tip of an ox goad that can enter it.
a) Rabbi Judah says: even if it cannot enter.
3) If the split was curved it must not be regarded as straight, and still the minimum size must be the circumference of the tip of an ox goad that can actually enter.
Section one: We have learned that if an earthenware vessel has a tightly fitting cover, impurity cannot enter into it and defile it. If such a vessel is found in the same room as a dead body, the vessel remains pure. In the case discussed here, if the netting (some sort of colander) is truly “tightly fitting” it counts as a cover and the vessel remains pure. If the part between the oven and the netting is split, if the split is the exact same measurement as the circumference of the tip of an ox goad, it is not considered tightly fitting, and the oven is impure (if it is in a tent with a corpse).
According to the first opinion, the tip need not actually be able to enter the split. Rabbi Judah holds that in order for the oven to be considered to not have a “tightly fitting lid” the split needs to be large enough to allow the tip to actually enter.
Section two: In this case, the split is not between the netting and the oven but in the netting itself. The opinions from section one are reversed.
We should note that there is no inherent logic as to when the measure is “the ox goad can enter” and when it is “even if it cannot enter.” It seems random why the sages reverse their opinions in the two cases. Albeck explains that all of the sages of the Mishnah (the tannaim) had a tradition that in one case the split must be large enough for the ox goad to actually enter, and in the other case, the split need be no larger than the ox goad. The sages and Rabbi Judah disagreed as to which measure was applicable in which case. In other words, they had some sort of halakhic tradition that they had received from their predecessors, but its details were not received and therefore they argued over them.
Section three: If the split was really like a small hole, from the oven to the netting, and it wasn’t just a regular crack, the minimum size still remains in effect. Albeck explains that this means that if we were to even out the curved split, and only then could it allow in the tip of the ox goad, the oven is still not considered to have a tightly fitting lid. It must actually be the size of the ox goad.