Kelim, Chapter Twenty-Five, Mishnah Nine
The final mishnah of chapter twenty-five gives a few general rules concerning the purity of vessels.
1) Holy vessels do not have outer and inner sides or a part by which they are held.
2) One may not immerse vessels within one another for sacred use.
3) All vessels become susceptible to uncleanness by intention, but they cannot be rendered insusceptible except by a change-effecting act, for an act annuls an earlier act as well as an earlier intention, but an intention annuls neither an earlier act nor an earlier intention.
Section one: The laws concerning holy vessels are more stringent than the laws governing regular vessels. Therefore, if one side of a holy vessel becomes impure, the entire vessel is impure. Similarly, the holding place is not considered separate from the rest of the vessel. If any part of the vessel is impure, even the holding place is impure.
Section two: It is possible to purify multiple non-sacred vessels by immersing one vessel within another one in a mikveh. But the laws governing holy vessels are more stringent. Each vessel must be immersed separately.
Section three: This section contains a crucial general principle with regard to the purity of vessels. Generally, a vessel is susceptible to impurity once its manufacturing has been completed. However, a vessel can become susceptible to impurity merely by intention, even if its manufacturing does not seem to be complete. For instance, if one has not fully completed the manufacturing of a vessel, but then decides that he is not going to work on it anymore and that he is going to use it as is, it is susceptible at that point to impurity (see 22:2, 26:5, 7). But if he has a vessel that is already susceptible to impurity and then he decides to modify it into a vessel that is not susceptible, it remains susceptible until he actually changes it. In other words, intent to use something as a vessel is sufficient to make it susceptible to impurity, but intent to change a vessel into a non-vessel is not sufficient.
The mishnah further clarifiesphysical changes wrought on a vessel can annul both the original manufacturing of the vessel or the intent the person had to use the vessel before it was ready. Put another wayphysically modifying a vessel can make it no longer susceptible to impurity (see 20:6, 26:9). However, intent cannot annul the original intent to use the vessel in a certain manner, or the original manufacturing. Intention is weaker than action (a good motto for life, if you ask me).