Mikvaos, Chapter 8, Mishnah 5. With this mishnah we enter the halachos of chatzitzah, the barriers that invalidate a tevilah. The Mishnah opens, however, with a case that turns on a different point entirely: not whether something blocks the water, but whether something in a person's mouth can pick up tumah from her while she immerses.
The Coins in Her Mouth
The spittle of a woman who is a niddah conveys tumah: whatever it touches becomes a rishon letumah. That only applies, however, once the spittle has parted from her, nekar, and has come to rest on something outside her. While it is still inside her mouth, counted as part of her own body, it conveys nothing.
"Niddah shenasnah ma'os lesoch piha veyardah vetavlah, tehorah mitumasan, aval temeiah hi al gav rukah." A niddah put coins into her mouth, went down to the mikveh, and immersed. Regarding the coins themselves she emerges tehorah: they sit loosely inside her mouth, fastened to nothing, so they neither block the water nor count as a chatzitzah. Her spittle is another matter, and it leaves her temeiah. Some of it inevitably settles onto the coins, and at that moment it has the status of nekar, having parted from her body, so it conveys tumah to her.
Hair, a Clenched Fist and Pressed Lips
"Nasnah se'arah befiha, kaftzah yadah, kartzah sifsoseha, ke'ilu lo tavlah." If she put her hair into her mouth, or clenched her hand into a tight fist, or pressed her lips shut tightly, her tevilah is invalid. The rule is that every place on the body must be ra'uy lebiyas mayim, fit for the water to reach. She is not required to hold her mouth open, but if she presses it firmly closed, or squeezes her hand shut, that place is no longer fit for the water, and it counts as a chatzitzah.
Holding a Person or Utensils While Immersing Them
"Ha'ocheiz be'adam uvekeilim umatbilan, temei'im." One who holds onto another person, or onto vessels, and immerses them that way, has accomplished nothing: they remain temei'im. The spot he is gripping does not allow the water in, and the object being immersed must be ra'uy lebiyas mayim throughout.
"Ve'im hediach es yadav bamayim, tehorim." If he first wet his hands in the water, they are tahor. The film of water between his hand and the vessel joins with the rest of the water of the mikveh, so there is no barrier at all, and the entire vessel counts as immersed. This is exactly the practice we follow when toveling keilim: the poskim direct that if a person is going to lower a vessel into the mikveh while holding it in his hand, he should first wet his hand.
"Rebbi Shimon omeir: yerapeh kedei sheyavo'u vahem hamayim." Rebbi Shimon holds that wetting the hand is not sufficient, since his grip may still be tight enough to seal the water out. In his view he must loosen his hold, so that the water can actually reach the place he is holding.
Beis Hastarim and Beis Hakmatim
The Mishnah closes with a rule about the tevilah of a person's own body. "Beis hastarim uveis hakmatim ein tzrichin sheyavo'u bahen hamayim." The hidden parts of the body, such as the inside of the ear, the inside of the mouth and the inside of the nose, and the folds of the body, places where skin lies over skin, such as beneath the arms, do not require that the water physically reach them. The source is the pasuk "verachatz es besaro bamayim," that he wash his flesh in water: what must be reached by the water is the flesh that is visible, not the concealed places within.