Mikvaos, Chapter 7, Mishnah 6. A kosher mikvah requires arba'im se'ah, forty se'ah of water. Our mishnah takes up the delicate situation of a mikvah that holds precisely that amount and not a drop more, where any loss of water immediately leaves the mikvah deficient.
Two People Immersing One After the Other
The opening clause reads "mikvah sheyeish bo arba'im se'ah mechuvonos", a mikvah whose contents come to forty se'ah on the nose. Then, "yordu shenayim", two men descended into it, "vetovlu zeh achar zeh", and each toveled in turn, the second entering directly after the first. On this the mishnah rules: "harishon tohor vehasheini tamei."
For the man who went in first there is no difficulty at all: at the moment of his tevilah the full forty se'ah were present, and he emerges tohor. The one who followed him gains nothing from his immersion and stays tamei. The reason is the water carried off on the first man's skin as he climbed out; that loss is enough to bring the mikvah below its shiur, so the second man toveled in a mikvah that was already lacking.
Rebbi Yehuda and Guda Achis
"Rebbi Yehuda omer: im hoyu raglov shel rishon nogos bamayim, af hasheini tohor." Rebbi Yehuda rules that if the first man's feet were still touching the water of the mikvah, the second man is also tohor.
The principle at work is guda achis: the water on the first man's body is regarded as still joined to the mikvah, drawn down and connected to it through his feet, which remain in contact with the water. Being connected, that water is mitztareif, it combines toward the measure, and the mikvah is considered a mikvah shaleim with its full forty se'ah.
A Cloth Still Touching the Water
From here the mishnah turns to a parallel scenario: "hitbil bo es hasagos vehe'elu miktzoson nogea bamayim, tohor." Someone dipped a tamei cloth in the mikvah, and the fabric soaked up mikvah water, so that part of the forty se'ah is now held within the material itself. Nevertheless, as long as some edge of the cloth has not left the water, the absorbed moisture counts as one body with the mikvah below. The shiur is therefore undiminished, and a person who immerses at that moment, with the cloth yet in contact with the mikvah, achieves taharah.
Among the meforshim there is a discussion here: is this halachah stated exclusively from the standpoint of Rebbi Yehuda, whose reasoning rests on guda achis, or does it hold up even for the other shittos?
The Pillow and the Cushion: A Practical Halachah
Finally the mishnah teaches a case of real everyday consequence: "hakar vehakeses shel or." Here we are not dealing with a plain garment but with a leather pillow or cushion, an item that has a receptacle, a hollow that holds water inside it.
"Keivan shehigbi'a sifsoseihen min hamayim", as soon as the rims of such an item are raised clear of the surface, whatever water sits within its hollow has been cut off from the body of the mikvah. Where the mikvah began with forty se'ah and nothing extra, it has now fallen short, because that trapped quantity in the kar or the keses no longer counts toward the shiur.
And the problem compounds itself. Once that water has been lifted out, it has the status of mayim she'uvim. Lower the pillow back in, and you are now pouring drawn water into a mikvah that is already short of forty se'ah.
So the mishnah asks "keitzad ya'aseh" - what should one do? The answer is beautifully practical: "matbilan uma'aleh osam derech shuleihen." Immerse them, and when removing them, bring them up bottom first, upside down. The water inside pours straight back into the mikvah before it is ever separated from it, so it never becomes she'uvim, the measure of the mikvah is never depleted, and the tevilah is valid.