Keilim, Chapter 17, Mishnah 16. Flat wooden utensils, ones that have no receptacle at all, are not susceptible to tumah. Our Mishnah presents a series of such utensils that were deliberately fitted with a concealed compartment, and in every one of these cases the compartment was installed in order to cheat somebody. The Mishnah opens with two such items and then goes on to list several more.
The beam of the balance scale
The opening item is "knei moznayim," the horizontal bar of a weighing scale. Goods in those days were weighed on a two pan device: a shopper who asked for a pound of flour watched the merchant set a one pound weight into one pan and pour flour into the other until both hung at the same height. Once the two sides evened out, everyone knew the quantity was exact. The bar stretched across the top is entirely smooth, a flat wooden implement with nothing that can hold anything, and such a utensil does not take on tumah.
The scraper
The second case is "vehamachok," a scraper. Volume was measured by filling a container of known capacity and then drawing a flat implement across the top so that nothing would be heaped up above the rim. Once the surface was level, whatever remained inside was precisely one measure. That scraper too is ordinarily a flat piece of wood with nothing to hold anything, and so it also is not susceptible to tumah.
The hidden metal compartment
But the Mishnah is speaking of a case of "sheyesh bahen beit kibul matechot," that these two items contain a concealed cavity for metal. The purpose of that cavity is dishonesty.
On the scale, the metal slides inside the beam to whichever side the cheater wants weighted down. If he is the buyer, he shifts the metal toward the side holding the weight. That side now hangs lower, and the seller has to keep adding flour until the pans finally even out, so the buyer walks away with more than he paid for. If he is the seller, he slides the metal toward the side holding the flour, so less flour is needed before the scale levels off, and the customer receives less than he purchased.
With the scraper the trick works differently. A scraper was normally fashioned from olivewood, a wood that is neither too heavy nor too light. Weight matters here: a scraper that is too heavy sinks down into the container and drags off too much of the flour, while one that is too light does not press down enough, leaving a small mound of flour standing above the rim. The cheater therefore carves his scraper from exactly the right wood, but hollows out its interior. When he is the buyer, he leaves the hollow empty. The scraper is now too light, it fails to press down, and extra flour remains in the measure for him. When he is the seller, he fills the hollow with metal. Now the scraper is heavy, it digs down into the flour as it passes over, and the buyer is left with less than a full measure.
The carrying pole
The Mishnah moves on to a further item, "veha'etzel," a shoulder pole, in which there is a "beit kibul ma'ot," a hidden space for coins. A person would rest such a pole across his shoulder with his bundle hanging from it. A dishonest laborer would carve a small cavity into the wood and tuck his money inside. When the workday was over and his employer asked whether the wages had already been handed over, the laborer would insist that he was carrying nothing at all. He would even invite a search of his person, and because the coins were concealed within the pole, the search produced nothing, so he was paid a second time.
The poor man's cane
The following item is the walking stick of a pauper, "vekaneh shel ani," fitted with a "beit kibul mayim," a hidden space holding water. This beggar would declare that he was in the middle of a fast, so that people would pity him and give more generously. Then, once no one was looking, he would sip the water concealed inside the stick and keep his strength up.
The staff with the mezuzah
The list closes with "umakel," a walking staff, hollowed out to hold a mezuzah together with pearls. This served as a scheme for slipping past the tax collector. The smuggler would drop his pearls into the cavity and set a mezuzah on top of them. If the official uncovered the compartment, its owner would display the mezuzah and claim that it is a sacred object exempt from duty, carried as a segulah for protection along the way. The pearls lying underneath escaped notice, and the treasury lost the tax it was owed on them.
The ruling
Concerning all of these the Mishnah rules "harei eilu temein," they are susceptible to tumah. Although each of these is a flat wooden utensil that on a Torah level does not receive tumah, once a concealed cavity has been fashioned in it we treat that cavity as a genuine receptacle, and the utensil can now become tamei.
Rabban Yochanan ben Zakkai's dilemma
The Mishnah's final line, "ve'al kulan," concerning every one of these devices, records what Rabban Yochanan ben Zakkai declared: "oy li im omar, oy li im lo omar." Woe to him should he teach these halachot, and woe to him should he leave them untaught. He was pulled in both directions. Were he to lay out these cases in detail, upright people who had never imagined such tricks would suddenly possess a manual for theft and deception. Yet were he to stay silent about them, the crooks already employing these devices would assume that the Chachamim had no grasp of what goes on in the marketplace, that the Sages knew nothing of the public's ploys and maneuvers. He wanted it clear that the Chachamim do observe what happens out in the world and understand its problems. Either path led to trouble, and Rabban Yochanan ben Zakkai was left without resolution as to whether these halachot ought to be taught.