Tevul Yom, Chapter 2, Mishnah 3. Our chapter is occupied with the practical question of what happens when a tevul yom, a person who has immersed in a mikveh but whose day has not yet ended, comes into contact with food and drink. Here the Mishnah places him at the side of a cooking pot and asks what his touch accomplishes, and then it steps back to compare his level of tumah with another rabbinic form of tumah, tumas yadayim.
The case opens: "Kedeirah shehi meleiah mashkin venaga ba tevul yom", a pot filled with liquid, and a tevul yom touched the liquid inside it.
When the liquid is terumah
"Im hayah mashkeh terumah, hamashkin pesulin vehakedeirah tehorah." Should the contents of the pot have the status of terumah, terumah oil, let us say, or terumah wine, the liquid itself becomes pasul, while the vessel holding it remains tahor.
Note the Mishnah's choice of word. It does not say the liquid became tamei; it says it became pasul. The liquid now carries the status of a shelishi letumah. That means it is itself disqualified, it may no longer be used as terumah, but it has no power to pass tumah along to anything else. This is precisely why the pot stays tahor: the liquid that the tevul yom touched has been ruined, yet it cannot carry the tumah any further to the vessel that holds it.
When the liquid is chullin
"Ve'im hayah mashkeh chullin, hakol tahor." If the liquid in the pot was ordinary chullin and not terumah, nothing at all has happened. A tevul yom does not convey tumah to chullin liquid, and so both the liquid and the pot remain entirely tahor.
If his hands were unclean
"Ve'im hayu yadav mesoavos, hakol tamei." If, however, the man's hands were unclean, then everything becomes tamei.
Tumas yadayim is a rabbinic institution. On the level of Torah law there is no such thing as a person who is tahor while one limb of his body is tamei; a body does not divide that way. But the Chachamim, out of concern for the purity of terumah, decreed in certain circumstances that a person's hands carry tumah even while the rest of him is tahor. When such hands touch the liquid in the pot, the liquid and the pot alike are rendered tamei.
Two comparisons
The Mishnah now draws the first of two comparisons: "Zeh chomer bayadayim mibitevul yom." In this case hands that bear tumah are the stricter of the two, since they accomplish a transfer of tumah that the tevul yom does not.
The comparison also runs in the opposite direction: "vechomer bitevul yom mibayadayim, shesefek tevul yom posel es haterumah, vehayadayim sefeikan tahor." Where a case of doubt arises, the tevul yom is treated more severely. If we cannot determine whether a tevul yom made contact with terumah, the halachah rules that the terumah has been disqualified. If we cannot determine whether hands bearing tumah made contact with terumah, the halachah leaves that terumah in its state of taharah.
The reason lies in the source of each tumah. The disqualification caused by a tevul yom rests on Torah law, and a doubt in a matter of Torah law is resolved stringently. Tumas yadayim is entirely a rabbinic decree, and a doubt in a rabbinic matter is resolved leniently. So each of the two carries its own chumra, and the Mishnah sets them side by side so that we should know exactly where each one is the stricter.