Keilim, Chapter 15, Mishnah 5. This mishnah turns to shovels made of wood, and it asks a simple question about each one: does this shovel become tamei or not? Some of these shovels were used for winnowing, that is, for separating the crushed beans from their husks. The worker would place the unprocessed beans into the scoop of the shovel and toss them upward. The lighter, unwanted part of the produce would be carried off by the wind, while the heavier kernels and beans would drop back down into the scoop. Our mishnah lists several kinds of wooden shovels and sorts out the tumah status of each.
The list opens with "Rachat shel garosot temei'ah": a shovel belonging to workers who grind beans can contract tumah. In their trade the hollow of the shovel does real receiving work, since they load the beans into it and then pour them out into the mill. Because the cavity is put to use holding the produce, the shovel qualifies as a wooden implement possessing a receptacle, and any implement of wood built that way is capable of becoming tamei.
"Shel otzarot tehorah". A shovel kept in a granary stays pure. Its job is of another sort entirely: grain lies spread out over the floor, and the worker drags the shovel along to draw the kernels together into one heap. He is pushing the grain, not transporting it. The cavity could certainly contain produce, yet containing is not the purpose for which this shovel exists, so tumah never takes hold of it.
"Shel gitot temei'ah". A winepress shovel is able to receive tumah. With it the worker clears away the skins and pits of the grapes, and in that task the hollow truly acts as a container, gathering those remnants so that they can be carried out of the press. Since the cavity is being used precisely for receiving, exactly as intended, this shovel is susceptible.
"Shel geranot tehorah". A shovel used on the threshing floor stays pure. The labor here is once again one of collecting: the grain lying about is pushed along into a single heap. The hollow is never pressed into service as a receptacle, so no tumah attaches to it, just as with the granary shovel. Granary and threshing floor are distinct locations, yet for this halachah the pattern of use is identical in both: nothing is ever held in the cavity, and where there is no receiving there is no tumah.
Now the mishnah states the rule that decides every one of these cases: "Ze haklal", this is the general principle, "he'asui lekabbalah tamei, lechinus tahor": whatever is fashioned for the sake of receiving is susceptible, while whatever is fashioned for the sake of collecting stays pure. A shovel built to contain produce can become tamei; a shovel built only to shift produce from one place to another remains tahor, notwithstanding a cavity entirely fit to hold things. The deciding factor is not what the shovel is able to do, but the purpose for which it was made.