Makhshirin, Chapter Five, Mishnah Ten
Today’s mishnah continues to deal with the purity of clean water that is being poured into impure water.
1) One who pours hot water into hot water, or cold water into cold water, or hot water [poured] into cold water remains clean;
a) But from cold water into hot water, the [cold water] becomes unclean.
2) Rabbi Shimon says: even in the case of hot water poured into hot water it becomes unclean if the strength of the heat of the lower [water] is greater than that of the upper [water].
Section one: If one pours from cold water into hot water, the heat of the impure hot water rises into the cold water and causes it to be impure. It seems that the sages think that the lower hot water might raise the temperature of the upper cold water. Since it has a physical effect, it also has a defiling effect.
However, in all other cases, the upper water remains pure. In the case of hot water being poured into hot water, since the upper water will not rise in temperature due to the lower water, it remains pure.
Again, simple contact through the flow does not cause the impurity to flow from below to above.
Section two: Rabbi Shimon says that if the lower one is hotter than the upper one, it too will defile the upper one, even if the upper one is also hot. The principle is that if the lower one could heat up the upper one, the impurity of the bottom contaminates the purity of the upper.