Kelim, Chapter Fourteen, Mishnah Eight
Our mishnah deals with the purity of keys.
1) A knee-shaped key that was broken off at the knee is clean.
a) Rabbi Judah says that it is unclean because one can open with it from within.
2) A gamma-shaped key that was broken off at its shorter arm is clean.
3) If it retained the teeth and the gaps it remains unclean.
a) If the teeth were missing it is still unclean on account of the gaps; if the gaps were blocked up it is unclean on account of the teeth.
b) If the teeth were missing and the gaps were blocked up, or if they were merged into one another, it is clean.
4) If in a mustard-strainer three holes in its bottom were merged into one another the strainer is clean.
5) A metal mill-funnel is unclean.
Section one: The first opinion holds that if a knee-shaped key is broken off at the knee, the key is pure because it can no longer be used to open the door.
Rabbi Judah says it still retains some function. The key can be used from the inside to unlock the door, but from the outside, the key would not reach the lock. Because it still has some use, it is still susceptible.
Section two: This key is similar to the Greek letter gamma. If it is broken off such that he can no longer handle it, it is clean.
Section three: This mishnah discusses keys in general, and not just the gamma key mentioned in section two.
A key has teeth and gapsboth are necessary to open the lock.
Obviously, if both remain, the key is functional and susceptible to impurity.
However, even if only the teeth or gaps are still there, the key is still susceptible to impurity.
The key only becomes clean when both the teeth are missing and the gaps are blocked up.
Section four: If there were three holes in a mustard-strainer that joined together to form one large gap, the strainer is no longer usable and it is clean.
Section five: The metal mill-funnel that is used for pouring wheat into the mill is considered a vessel and is susceptible to impurity.