Ohalos, Chapter 1, Mishnah 2. The opening mishnayos of this masechta count how far the tumah of a meis can travel from one thing to the next, and how many links such a chain can have. The first mishnah described a chain of two: Reuven touches the meis, Shimon touches Reuven, and there the line stops. There is no Levi in that picture. Our mishnah now asks about a longer chain, one of three links, and shows us the case in which it happens.
The mishnah asks: "Keitzad shloshah?" How do we arrive at a count of three? And it answers: "Keilim hanogim bameis", vessels that touch a corpse.
The mishnah poses its question: "Keitzad shloshah?" By what path does the tally reach three? Its answer is brief: "Keilim hanogim bameis", utensils that come into contact with a corpse.
Here lies the chiddush of the mishnah. Our instinct tells us that contact with a corpse always costs one rung. The body is the most severe grade there is, avi avos hatumah, so whatever touches it ought to emerge one step lower, as an av hatumah. For human beings that instinct is precisely correct: a man who comes into contact with a corpse becomes an av hatumah while the body retains its own grade. A utensil, however, follows a different rule. Contact with the corpse costs it nothing at all in status. It stands together with the body itself, at the grade of avi avos hatumah.
Hence the mishnah's formulation: "Keilim hanogim bameis", utensils in contact with the corpse, hold the very grade the corpse holds, avi avos hatumah. "Vekeilim bekeilim", and a further utensil that receives tumah from the first one drops a rung and emerges as an av hatumah. It is at this second stage that the decline in status finally sets in: the utensil that met the body ranks alongside it as avi avos hatumah, while the utensil that received from it holds no more than av hatumah.
Both of these first two links carry the full severity: they are tamei tumas shivah, tamei for seven days. That also means they require the process of the parah adumah, with the sprinkling of the ashes on the third day and the seventh day.
The Third Link
Now comes the third step: "Vehashlishi, bein adam bein keilim, rishon letumah utmei tumas erev." Whoever comes next in the chain, whether it is a third set of keilim touching the second set, or a person who touches the second keli, is a rishon letumah and is tamei only until the evening.
It is worth tallying the stages one by one. Stage one is the utensil in contact with the body, and its grade is avi avos hatumah. Stage two is the utensil that received tumah from it, which stands as an av hatumah. Stage three is whoever comes next, whether a person or a further vessel, and that one holds the grade of rishon letumah, tamei for that day only until nightfall. The first two stages, holding the grades of avi avos hatumah and av hatumah, bear a seven day tumah; the third is far lighter and calls for no sprinkling of the ashes of the parah adumah at all.
Where Is This Learned From?
The natural question is what tells us that a utensil in contact with a corpse holds its grade rather than descending the way a person does. The source is a verse in the Torah: anyone who, out in an open field, comes into contact "bachalal cherev o bameis", with one slain by a blade or with a corpse, bears a tumah of seven days.
On its face the word "cherev", a sword, is superfluous. A corpse conveys tumah whatever brought about the death, be it violence or illness, and the verse in any event goes on to say "o bameis" explicitly. The blade is named in order to establish a rule: "cherev harei kechalal", the weapon carries the same din as the victim. Chalal denotes a body from which the neshamah has departed, and the verse sets the sword on identical footing with it: since a body emptied of its neshamah holds the grade of avi avos hatumah, so does the blade that met it.
Does this halachah govern only a literal blade? The Bartenura maintains that other utensils are covered as well, among them wooden implements fit to be toveled in a mikveh, and even clothing. Earthenware, a kli cheres, is the single exception. Apart from it, every vessel that comes into contact with a corpse holds the grade of avi avos hatumah. There are Rishonim, however, who confine this halachah to vessels of metal.
Summing Up
The case before us, then, is a utensil (of metal, or of other materials as well according to the views cited) that came into contact with a corpse. That utensil is stage one, and its grade is untouched: avi avos hatumah. From there it conveys tumah to a second utensil, stage two, which comes down to av hatumah. That second utensil in turn reaches a person or a third vessel, and the recipient at stage three, human or utensil alike, is still tamei but has descended to rishon letumah. The two earlier stages, holding the grades of avi avos hatumah and av hatumah, bear a seven day tumah, while the last is tamei for that day alone, until evening falls.