As we learned in the first Mishna in this perek (chapter) (see 20b), great care must be taken to ensure ritual purity in the cases of teruma (tithes) and kodashim (Temple sacrifices), but the demands made regarding kodashim are greater than those having to do with teruma. For example, if two vessels are both tameh – ritually impure and must be immersed in a mikveh – for the purposes of teruma they can be immersed even when one is inside the other. For use with kodashim, however, they would have to be immersed separately.
Why must the vessels be immersed separately for kodashim? Two suggestions are offered in the Gemara:
Rabbi Ila suggests that it is a concern with hatzitza – that the weight of the inner vessel may cause it to touch the outer one, thus preventing the mikveh waters from properly coming into contact with both vessels as is essential for them to be purified. The Talmud Yerushalmi, which offers this as the only problem with immersing the two vessels together, also gives a specific weight – a litra – that creates such a concern.
Rava suggests another potential issue with immersing the two vessels together. He proposes that the problem stems from a concern lest someone try to immerse small objects, like pins or needles, within a vessel whose opening is so small – smaller than a shfoferet ha-node (the tube of a skin bottle) – that the water inside of it is not considered connected to the larger mikveh.
The node of a shfoferet ha-node is a bag or bottle made of an entire skin removed from an animal. These skins were used for a variety of purposes, but primarily to store small objects or food. When one was used to store liquids (water, wine or oil, for example) the skin would be removed without making any holes in it, and they would leave the skin of the legs attached, as well. When finished, one of the legs would have a tube – usually a reed – inserted into it, and the liquids would be poured in and out from that small tube.