Keilim, Chapter 1, Mishnah 2. The opening mishnah of the masechet listed avot hatum'ah that convey tum'ah through maga, direct physical contact. Our mishnah continues the list and climbs a step higher: it presents avot hatum'ah that convey tum'ah in a stricter manner, not only by touch but even through masa, through being carried. Carrying here means that a person is bearing the weight of the tamei item, even in a case where he is not actually touching it. That form of transmission does not exist by the items of the previous mishnah, and that is precisely what makes these avot hatum'ah more severe.
The mishnah therefore opens: "lema'alah meihem", higher than them, meaning that the avot hatum'ah about to be listed carry a greater power of transmission than those listed before them.
Neveilah and Mei Chatat
The first is "neveilah", the carcass of an animal that died without kosher shechitah.
The second is "u'mei chatat sheyesh bahen kedei hazayah", the purification waters of the Parah Adumah, the mixture of the ashes with the water, in a quantity sufficient for a sprinkling. That is the amount needed for sprinkling upon a person or upon a keli that had contracted tum'at met, tum'ah from a human corpse. Less than that measure does not carry this stringency; the mishnah is speaking of waters that still hold enough for a hazayah.
"Shehen Metam'in et Ha'adam Bemasa"
Why are these two more stringent than the avot of the previous mishnah? "Shehen metam'in et ha'adam bemasa", because they render a person tamei through carrying. A person who bears their weight becomes tamei even without any direct contact with them.
With regard to a neveilah there is a further point: even moving it, pushing it or dragging it, which is called heset, renders the person tamei.
The scope of this law needs precision. Notice that the mishnah singles out a human being: "et ha'adam". It is the one who lifts and bears such an item, whether the animal carcass or the sprinkling waters, who is drawn into tum'ah. Vessels, however, along with foods and liquids, kelim, ochalin and mashkin, are not affected this way. Being transported in the company of these avot hatum'ah, or sharing a load with them, leaves them exactly as they were.
Picture a man carrying a stack of layers upon his back, with a piece of a neveilah resting at the very top of the pile. The man himself becomes tamei, because he is bearing the weight of the neveilah, even though nothing of it is touching him. The uppermost keli, the layer in actual contact with the neveilah, also becomes tamei, because it is touching it. But everything in between, all the intermediate layers, remain tahor, since they are neither touching the neveilah nor affected by the fact that they are being carried with it.
"Letamei Vegadim Bemaga"
The mishnah adds a further stringency: "letamei vegadim bemaga". A person in the act of carrying a neveilah or mei chatat conveys tum'ah to garments and to other utensils by touching them.
To appreciate this we need the basic vocabulary of Seder Taharot. The items listed in the mishnah are themselves an av hatum'ah. A person who contracts tum'ah from an av hatum'ah becomes a rishon letum'ah, one level removed from the source. As a rule, a rishon letum'ah cannot convey tum'ah to kelim. If such a person touches a utensil, the utensil remains tahor.
Here, however, there is a special law. So long as the person is actively bearing the av hatum'ah, any keli that he touches at that moment does become tamei, and it too becomes a rishon letum'ah, even though the person himself is only a rishon. Return to our image: the neveilah lies on his back and is not touching his body at all. Because he carries it, he becomes tamei and takes on the status of rishon. Ordinarily, anything he touches afterward would stay tahor, since a rishon does not convey tum'ah to utensils. But while he is actually carrying, befo'al, whatever he touches becomes tamei and becomes a rishon letum'ah like him.
"Vechasuchei Vegadim Bemaga"
The mishnah ends with the reverse case: "vechasuchei vegadim bemaga", clothing is held back from this tum'ah where there was contact and nothing more. Should the person have only laid a hand on one of these tamei items rather than lifting them, whatever garments or vessels he goes on to handle stay tahor. Even the very clothes on his body at the instant he touched the carcass or the sprinkling waters are unaffected, and a keli he happens to be holding is unaffected as well. Clothing and vessels are pulled in only where the man's own tum'ah came from bearing the item's weight, and never where it came from touch by itself.
Our mishnah has thus taught us two avot hatum'ah, neveilah and mei chatat in the measure fit for sprinkling, which stand a level above those of the preceding mishnah: they render a person tamei even when he only bears their weight, and while he bears them he transmits tum'ah onward to garments and utensils that he touches.