Mikvaos, Chapter 10, Mishnah 1. This perek turns primarily to the halachos of tevilas keilim, the immersion of vessels in a mikveh, and the opening mishnah begins with yados hakeilim, the handles of vessels, and then moves on to the problem of air trapped inside a vessel during its immersion.
The mishnah opens with the words "Kol yados hakeilim", a phrase that takes in every kind of handle attached to a keli. Picture a utensil built with a socket into which a handle is meant to sit. Three situations are laid out. "Shehichnisan shelo kdarkan": the handle was pushed in improperly, never really settled where it belongs. "O shehichnisan kdarkan velo mir'akan": its placement was correct, yet it was never forced in snugly, so it wobbles. "O shemir'akan venishberu": it had been seated firmly and properly, but it later snapped in a way that leaves it useless as a handle.
Concerning all three the mishnah rules "harei eilu choitzin", they interpose. Since in each of these cases the handle is not serving in the vessel's normal use, it is not considered part of the keli but rather an extraneous item attached to it, and it forms a barrier between the vessel and the waters of the mikveh.
The mishnah now moves to a different obstacle to a proper tevilah, one made not of matter but of air. "Keli shehitbilo derech piv, ke'ilu lo taval." A vessel with an inner cavity, a bottle or a jug, which the owner turned upside down so that its mouth faced downward, and in that position lowered it into the mikveh. A vessel that appears to be empty is not truly empty: it is full of air. When it is pressed mouth downward into the water, that air cannot escape, and the water is therefore unable to enter. The inside of the keli never meets the mikveh at all, because the trapped air stands between the water and the walls of the vessel. Such a tevilah is as though it never took place.
Regarding this halachah, the Mechaber writes in Shulchan Aruch, Siman 202, se'if 6, that the difficulty arises only by a keli whose neck and mouth are constricted; when the opening is broad, the air makes its way out by itself and nothing prevents the tevilah. The Vilna Gaon there, in se'if katan 6, takes the other view, and just how large an opening needs to be before this issue falls away is left as a machlokes haposkim. What Yidden actually do, therefore, is to lean the keli over onto its side so that every bit of air is released, which is precisely the course the mishnah itself goes on to prescribe, and that is our practice with any vessel that has an opening.
"Hitbilo kdarko belo ziboyrus." He immersed the vessel in the ordinary manner, standing upright with its base downward, but he did not bring its neck fully beneath the surface, and he did not tip it over. The mishnah's ruling is "ad sheyatehnu al tzido", the tevilah is not valid until he tilts the vessel onto its side. The concern is the same: with a narrow mouth, the air inside will not let the water in. Once the keli is tipped sideways, the air escapes, water takes its place, and only then is the vessel wholly immersed in the mikveh.
A parallel case follows. "Keli shehu tzar mikan umikan verachav ba'emtza": a vessel that is narrow at both ends, with openings on either side, and wide in the middle. Here too "eino tahor ad sheyateinu al tzido", it does not become tahor until it is turned on its side so that the air can work its way out and the water can fill the broad center.
"Tzelochis shepiha shaku'a": a small flask whose lip is sunken and folded inward, creating a pocket of air within that recess. "Eina tehorah ad sheyinkevenah mitzidah", it does not become tahor until one pierces it at the side and makes a hole through which the water can reach that hidden space during the immersion, for otherwise we are concerned that the water will never enter there.
The mefarshim raise a difficulty here. We have a principle that beis hastorim, the concealed areas of a vessel, do not require bi'as mayim, and do not even need to be re'uyah levi'as mayim, fit for water to reach them. By the tevilah of a person, hidden places must at least be capable of receiving the water; by a keli, that requirement does not apply. If so, why does our mishnah imply that these enclosed pockets in a vessel must be opened to the water? This remains a question, and there is discussion among the poskim whether this mishnah is halachah lema'aseh at all. The Mishnah Achronah likewise takes issue with the matter.
"Kalmarin hedyoytis": an ordinary person's inkwell, which is built with an inturned rim or lip on top so that the ink should not spill out. "Eina tehorah ad sheyinkevenah mitzidah." Precisely because its opening is tight and it is designed so that nothing should escape from it, when it is immersed the water will not get inside. The only way to render it tahor is to pierce it at the side, making a hole through which the water can enter that area as well.
The mishnah concludes with a historical illustration. "Kalmarin shel Yosef HaKohen haysa nekuvah mitzidah." The inkwell belonging to Yosef HaKohen was pierced at its side, and for that reason, when it was immersed, its tevilah was a valid ma'aseh tevilah, since the water was able to reach within.