Taharos, Chapter 3, Mishnah 1. We now open a new perek, and it takes up a subject that is genuinely fascinating: foods that sit on the border between solid and liquid, and what happens to their tumah status when they cross from one side of that border to the other. Since solids and liquids are treated very differently in the laws of tumah, a food that changes phase raises a real question.
Three Borderline Foods
The mishnah lists them: "Harotev vehagrisim vehechalav" - gravy, a dish of ground beans, and milk. To us, living with refrigeration, milk may not seem borderline at all, but it certainly is: milk has every capacity to thicken and set. Anyone who enjoys butter understands the point. Each of these three can be found as a flowing liquid or as a congealed mass.
When They Are Liquid
"Bizman shehen mashkeh tofe'ach, harei eilu techilah." The test the mishnah gives is moistness: as long as the substance is wet enough that touching it leaves your hand damp, it counts as a liquid. And once it is a liquid, the familiar rule applies, the one we have met repeatedly: a liquid that contracts tumah becomes a techilah, a first-level tumah. Liquids do not sit at the level they received; they jump to rishon.
When They Congeal
"Karshu, harei eilu sheniyim." Suppose the substance thickens and sets. It no longer moistens the hand, so it is now a solid, and a solid is a new entity that does not simply carry over the properties of what it was before. So how do we rate it? The mishnah says it is a sheni, and the reasoning is beautiful.
Picture the broth slowly setting. The first bit that hardens is, at that instant, a newly formed solid food, and it is sitting directly against liquid that is a rishon. Contact with a rishon makes it a sheni immediately. As the next portion congeals, it too is formed right up against rishon liquid, and it too becomes a sheni on the spot. Point by point, throughout the entire process of setting, every part of the emerging solid acquires the status of sheni. That is the rule for anything moving from liquid to solid.
When They Melt Back
"Chazru venimochu" - and now the congealed food turns back into liquid. Once again the liquid is a new entity and does not automatically inherit the status of the solid it came from. So what is its status?
Here everything hinges on quantity, and the mishnah gives the measure: "kabeitzah mechuvan, tahor; yoser mikabeitzah, tamei." If the congealed mass was exactly a kabeitzah, or less, the melting liquid is tahor. If it was more than a kabeitzah, the liquid is tamei.
The answer rests on a yesod taught back in the opening mishnayos of Maseches Taharos: an amount of food that falls short of a kabeitzah is fully able to receive tumah, yet it lacks the strength to transmit it onward. Only a mass measuring at least a kabeitzah has the power to render something else tamei.
Now apply that. If the solid is precisely a kabeitzah, then the moment the first drop melts away, what remains of the solid is already less than a kabeitzah, and it can no longer convey anything. There is never a point at which there is enough solid present to render the emerging liquid tamei, so the liquid stays tahor.
But if the mass is larger than a kabeitzah, then when that first drop melts there is still a kabeitzah or more of sheni solid standing there, fully able to convey tumah, and the drop becomes tamei. And once that first drop is tamei, the rest follows: as the mass continues to melt, even after it shrinks below a kabeitzah, each new drop joins the liquid that is already a rishon and becomes part of it.
This is exactly the mishnah's own explanation of its ruling: "shekeivan sheyatza tipah harishonah, nitm'ah bekabeitzah" - since the first drop emerged while a full kabeitzah of tamei solid was still present, that drop was rendered tamei by it, and the fate of the entire melting mass is sealed by that first moment.