Why does the bottom cup dominate? If hot is equally poured into cold or the opposite, by what method does this seemingly unscientific law hold? This one has perplexed us for quite a while and we look forward to your answer.
Barry Epstein
(a) According to TOSFOS, our Sugya is not discussing the concept of Tasa'a Gavar at all, and it is not discussing pouring equal quantities into each other. Rather, the Gemara is discussing pouring a small amount of water into a larger amount (on the bottom). (See Insights)
(b) According to Rashi, who understands that the Sugya is discussing Tasa'a Gavar, we mentioned in the Insights the SEFAS EMES who says that this is not the normal Tasa'a Gavar that we find elsewhere, because the normal Tasa'a Gavar does not apply to two liquids that become mixed with each other. Rather, here we are discussing a case where the bottom liquid is considerably greater in quantity that the one that was poured into it (like Tosfos). (Even though we wrote that the quantities are approximately the same according to Rashi, it could be that he indeed understands that the bottom quantity is larger than the one from the top.)
(c) The normal concept of Tasa'a Gavar (discussed in Maseches Pesachim 76a) refers to pouring a solid onto a liquid or a liquid onto a solid.