This Mishnah will have a machloket (disagreement) between Rabbi Tzadok and the Hachamim (the Sages) about whether certain objects are susceptible to becoming tamei (ritually impure) or not. The Mishnah says: There are three different things that Rabbi Tzadok says could become tamei, but the Hachamim disagree and say these things are incapable of becoming tamei.
Number one, the nail of a money changer. A money changer had scales. He would weigh the money to understand its value using proper weights, and he would hang the scale on a nail attached to a cabinet. The halacha (Jewish law) is that a metal object, even if it does not have a beit kibbul, a hollow space, could become tamei if it serves people’s needs. For example, a pair of scissors does not have a beit kibbul. It does not have a hollow space to store something. It is not a keli (utensil) in that regard, but it has a use, so therefore it could become tamei. Rabbi Tzadok also connects that to this nail where the scales are hung. According to him, it serves people’s needs, so therefore it is a utensil and could become tamei. The Hachamim disagree and say that the nail does not have its own function; rather, the nail stuck into the cabinet is part of that cabinet, and it could only become tamei if the cabinet becomes tamei. That is number one.
Number two, an aron, a chest of a bean grinder. Bean grinder, he stores the beans that he ground up into flour in a wooden chest. That wooden chest is usually kept in one spot and does not move around. According to the Hachamim, the rule is that a wooden object cannot become tamei unless it is usually moved from place to place. Therefore, this wooden chest, which is kept in one spot, cannot become tamei. But according to Rabbi Tzadok, it is occasionally moved to repair it and for various other reasons. Although it is moved around rarely, being moved once in a while is enough for it to be considered a keli, and therefore it could become tamei.
Number three, the nail of a sundial. The sundial was what they would use in those days to tell time. They would have a nail right in the middle of it, and the shadows cast by that nail would point to the right hour of the day. According to Rabbi Tzadok, since the nail is used, it has a purpose, like the nail used by the money changer. It serves human needs, and therefore it could become tamei. But the Hachamim disagree and say, “No, really, anything could make a shadow. We happen to be using a nail in this situation. Since anything could be used, it is not such a big deal that it was a nail. Therefore, it is not enough for it to be classified as a utensil.” According to the Hachamim, it cannot become tamei. It always remains tahor (pure), and it is not considered to be a utensil.
Rabbi Tzadok metamei (declares impure), and the Hachamim metaharin (declare pure). In all of these cases, Rabbi Tzadok says that the item is a utensil capable of becoming tamei, but the Hachamim disagree. They say that these items remain pure, tahor. They cannot become utensils.