Keilim, Chapter 23, Mishnah 5. Our Mishnah turns to the world of the fisherman and the trapper, and asks which of their tools are considered true keilim that can become tamei, and which are not.
To follow the Mishnah we first need to picture how a fishing net was built. It had two parts. At its base was a piece of woven cloth, and through that cloth the ropes were threaded and knotted to form the netting that actually caught the fish.
Each part has a different status. The cloth base is a garment, and a garment is subject to tumah. The rope netting, however, cannot become tamei on its own. It is not treated as a woven cloth, since it is mostly open space, gaps between one rope and the next. Nor is it treated as a receptacle, because the fish that swim into it can find their way back out through those same openings. Something that holds its contents so loosely that they escape is not a container.
The large net
The Mishnah's first ruling is "hacherem tamei mipnei hazoto". The cherem, a broad net spread out to catch fish, does receive tumah, and what makes it so is the strip of woven material along its lower edge. The rope mesh by itself could never become tamei, but net and cloth together form one object. So the moment the cloth portion becomes tamei, the tumah takes hold of the entire utensil, mesh and all.
The traps that are temei'in
"Hareshatot vehamichmarot" - smaller nets and traps, which genuinely enclose the animal, bird or fish that has been caught inside them.
"Hamadaf" - a snare set for birds. A board was propped up on a stick over a small basket, with food placed underneath to draw the bird in. When the bird came to snatch the bait, it knocked the stick loose, the board came down, and the bird was left trapped in the basket.
"Vehapaltzur" - a wooden trap in which animals were held.
"Umtzudot hasakarin" - the traps of those who dam up rivers. The fishermen would divert the current so that the fish were carried into a small pool. They would then drain the pool, and the fish would be left lying at the bottom to be gathered up.
Regarding all of these the Mishnah rules temei'in, they can become tamei. Each one is a real receptacle: it holds what is inside it, and it is used to carry the catch from place to place.
The traps that are tehorin
"Veha'akun" - a fish trap shaped like a chest and lined on the inside with hooks. The fisherman would sink it beneath the surface, and fish that swam in were caught on the hooks.
"Veharetov" - a device for catching birds. "Vehakeluv" - a cage in which a bird would be caught.
These three the Mishnah declares tehorin, since none of them is truly a vessel. The akun holds nothing inside it at all: the fish remain hanging from the hooks rather than resting within. As for the retov and the keluv, no one carries birds about in them; they serve only to seize the bird where it lands.
This is the principle that runs through the whole Mishnah: for a wooden utensil to be susceptible to tumah, it is not enough that it be able to hold its contents. It must also serve to move them, to be carried from place to place with what is inside it. A device that only seizes and detains, without transporting, remains tahor.