Machshirin, Chapter 3, Mishnah 3. This chapter deals with produce and food that become wet without anyone pouring liquid on them, and with the question of whether such wetting counts as beratzon, so as to render the food hechsher lekabel tumah, ready to receive tumah. Our mishnah discusses a loaf of bread taken hot from the oven and rested on the mouth of a barrel of wine.
Here is the wording of our mishnah: "Harodeh pas chamah venasnah al pi chavis shel yayin", meaning that someone lifts a still steaming loaf out of the oven and lays it across the opening of a wine cask. About such a case the Tannaim differ: "Reb Meir metamei, Reb Yehudah metaher."
Why the bread was tahor until now
The scenario presupposes a loaf whose dough was worked with fruit juice rather than with water. Normally water is used, and then the dough has already met one of the seven liquids, so the finished bread carries hechsher lekabel tumah from the moment it was kneaded. With fruit juice, which is not among the liquids that produce hechsher, nothing in this loaf has yet given it the capacity to receive tumah. The entire issue is created by the placement: the loaf leaves the oven hot and is laid over the wine, and its heat may pull some of that wine up into it. Would such wine confer hechsher lekabel tumah upon the loaf? In Reb Meir's view it would, while Reb Yehudah rules that it would not.
A second way to read the dispute
That is the first explanation offered by the Rav. He offers a second as well. Perhaps the mishnah is speaking of a barrel whose wine was tamei, and the question is whether the loaf itself becomes tamei on account of that tamei wine: does the bread absorb the liquid lying beneath it, so that the tumah is conveyed to the bread? Here too, Reb Meir says that it does and Reb Yehudah says that it does not.
The existence of these two readings points to an imprecision that appears throughout this perek. When a Tanna is described as metamei, is the claim that the item turns tamei outright, or only that it has now become susceptible, having acquired hechsher lekabel tumah? On the Rav's opening approach, the machlokes concerns nothing more than whether the loaf was wet at all and thus became susceptible. On his alternative approach, real tumah is at stake: the tamei wine sitting beneath the loaf either transmits its tumah into the bread or it does not.
Wheat bread and barley bread
A third opinion closes the mishnah. "Reb Yosi metaher beshel chittin": a loaf of wheat he rules tahor. "Umetamei beshel se'orin": a loaf of barley he declares tamei. And his reason follows: "mipnei shehase'orim sho'avos", because barley soaks liquid up more readily than wheat. Since everything hinges on whether the loaf genuinely drew the wine into itself, Reb Yosi holds that barley absorbs while wheat does not.
The mishnah therefore leaves us with three positions on a single practical scene: a hot loaf resting on an open barrel of wine, and the halachic status that depends entirely on how much of that wine the bread actually takes in.