Machshirin, Chapter 5, Mishnah 10. This chapter has been dealing with water and with the ways in which tumah does or does not travel from one place to another through liquid. The immediately preceding mishnah taught the rule of a person pouring from a tahor vessel into a tamei vessel: the descending stream is not treated as a connection between the two vessels, and so the tumah of the lower vessel does not climb back up the stream to render the upper water tamei. There were exceptions to that rule, such as thick honey, where the substance being poured is heavy enough that the upper and lower liquids really are one, but as a rule the stream is no bridge for tumah.
Our mishnah now looks at the same act of pouring from a different angle: not the stream itself, but the heat of the vessels and the steam that rises between them.
"Hame'areh mecham lecham, umitzonein letzonein, umecham letzonein, tahor." Three combinations are listed, and all three end with the same verdict. Whether one empties hot water down into a utensil that is likewise hot, or cold into cold, or hot water into something cold, whatever liquid remains above keeps its taharah. In none of these three does the tamei receptacle underneath have any effect on the water still in the upper utensil.
"Mitzonein lecham, tamei." The fourth combination reverses the outcome. Picture cold water being emptied into a utensil that is hot, where the upper utensil is tahor and the lower one is tamei. Since all the heat is below and none of it above, vapor climbs out of the lower utensil into the liquid overhead, and that ascending vapor conducts the tumah of the lower utensil upward, so that the water above becomes tamei. The route taken by the tumah here is not the falling stream but the rising steam.
Rav Shimon disagrees about one of the earlier cases. "Af hame'areh mecham lecham," he says: even hot poured into hot can end in tumah, "vekoach tachton yafeh," where the strength of the heat below is better, that is, "mishel elyon, tamei," greater than the heat of the utensil above, the water in the upper utensil is tamei. His argument follows the same reasoning as the previous ruling: when the tamei utensil underneath is the warmer of the two, vapor will still rise out of it into the liquid above it and carry the tumah along.
We do not rule in accordance with Rav Shimon. His logic is perfectly understandable, yet when both vessels are hot, the greater heat of the lower vessel is not nikar, not noticeable enough to be reckoned with. A difference in heat that cannot be discerned is not treated as producing a stream of steam capable of conveying tumah, and so in that case the upper water stays tahor.
The principle to carry away from this mishnah is that steam rising from a tamei vessel into water above it can serve as the channel through which tumah ascends, but only where the heat below clearly outstrips the heat above: cold poured into hot. Where both vessels share the same state, hot into hot or cold into cold, or where the heat runs in the other direction, hot into cold, the water being poured remains tahor.