Rambam - 1 Chapter a Day

Whenever a person makes a receptacle, it is susceptible to impurity according to Scriptural Law regardless of its size. There is no minimum measure for a receptacle, provided it is made from a lasting substance that will be maintained.

What is implied? When a person makes a utensil from a hide that was not processed at all, from paper, even though the paper itself is not susceptible to impurity, or from the shells of pomegranates, nuts, or acorns, even if children hollowed out the shells to measure earth or they were prepared to be used for scales, the utensils are susceptible to ritual impurity. The rationale is that the deeds of a child, a deafmute, a mentally or emotionally compromised person and a minor are halachically significant, although their intent is not halachically significant.

If, however, one makes a utensil from a dried turnip, esrog, or squash, hollowing them out to measure with them or the like, they are pure, for it is impossible that they will last longer than a short while.

All of the following - the horizontal rod of a scale and a leveling rod that have a receptacle where metal can be placed, a peddler's pole that has a receptacle where coins can be stored, a wooden pole that has a place where water is stored, a staff that has a receptacle for a mezuzah or a place for a pearl, a wooden sharpener that has a receptacle for oil, a wooden writing tablet that has a receptacle for wax - are susceptible to ritual impurity according to Scriptural Law, because they have a receptacle, even though it may be of the smallest size.

According to Scriptural Law, it is only the receptacle that these keilim contain and the part of the k'li that serves the receptacle and that the receptacle requires that is susceptible to impurity. The remainder of a flat k'li that is not necessary for the receptacle, is pure according to Scriptural Law. It is susceptible to impurity according to Rabbinic Law, as we explained.

When a receptacle is intended to be filled, it is not considered as a receptacle. What is implied? When a receptacle is hollowed out from a block of wood and then a metal anvil is inserted into it, if it is a blacksmith's, it is not susceptible to ritual impurity. Although it has a receptacle, it was made to be filled. Similar laws apply in all analogous situations.

If the anvil was for jewelers, the k'li is susceptible to ritual impurity, because whenever they desire, they lift up the iron and collect the filings of gold and silver that collect there, under the anvil. Thus the hollow is meant to serve as a receptacle. Similar laws apply in all analogous situations.

When a cup is carved below the legs of beds, chests, and the like, even though it is a receptacle, it is pure. It is considered as if it does not have a receptacle, because it is not intended to collect anything, but rather to support furniture.

A hollow piece of straw is susceptible to impurity like any wooden k'li that is susceptible to impurity even if it can only take in one drop. A hollow reed is not susceptible to impurity until all the white sap in it is removed. If it was not cut for the sake of taking in other substances, it is considered like other flat wooden keilim. The hollow stems of gall and the like are not considered as keilim, but rather as foods.

When one cuts a straw, inserted a mezuzah inside of it, and then placed it inside a wall, it is susceptible to impurity even if he placed it in the wall with its open part pointed downward. If he affixed it to the wall, affixing it with its open part pointed upward, it is susceptible to impurity. If the open part is pointed downward, it is pure.

Should one place the straw in the wall and then insert the mezuzah inside, if the open part was pointing upward, it is susceptible to impurity. If the open part is pointing downward, it is pure. If he affixed it to the wall, even if its open part is pointing upward, it is pure.

The following laws apply to a k'li that is made by weaving pieces of wood or sh'am for the purpose of spreading clothes over it while incense is burned below it so that they become fragrant. If it was made like a beehive that has no base, it is pure. if it has an opening where a covering can be placed, it is susceptible to impurity.

A metal foot-covering for an animal is impure. If it is made from sh'am, it is pure, because that is not considered as a receptacle.

When a person bundles a pearl in a hide and, after he removes it, a hollow is left, it is susceptible to impurity until it is smoothed out. For any container is considered a receptacle if it can hold even the slightest amount and, in the above instance, something resembling a small pocket is left. If, however, one bundles coins in a hide, it is not susceptible to impurity, because such a bundle does not have the form of a container at all.