Pesachim 76A

אסור. צונן לתוך צונן ⁠— דברי הכל מותר. חם לתוך צונן, וצונן לתוך חם, רב אמר: עילאה גבר. ושמואל אמר: תתאה גבר.

that the permitted foods become forbidden, because they absorb some of the forbidden food. If a cold food item falls into another cold item, all agree it is permitted; the food needs only to be rinsed off. The dispute pertains to a hot food item that falls into a cold one or a cold food item that falls into a hot one. Rav said: The upper one prevails. The halakha is determined based upon the state of the upper substance. If the upper food is hot, the case is judged as though a hot food fell into another hot food because the upper food heats the lower food. If the upper food is cold, the case is similar to a situation where a cold food falls into another cold food because the upper food cools down the lower one and prevents absorption. And Shmuel said: The lower one prevails. In his opinion, if the upper substance is hot and the lower one is cold, the permitted food remains permitted; if the lower one is hot and upper one is cold, they are forbidden.

תנן: נטף מרוטבו על החרס וחזר אליו ⁠— יטול את מקומו. קא סלקא דעתך בחרס צוננת. בשלמא לרב דאמר ״עילאה גבר״, משום הכי יטול את מקומו ⁠— דאזל רוטב מרתח ליה לחרס, והדר חרס מרתח ליה לרוטב. וכי הדר רוטב אפסח ⁠— קא מטוי פסח מחמת חמימותא דחרס, ורחמנא אמר ״צלי אש״, ולא צלי מחמת דבר אחר.

We learned in the Mishnah: If some of the gravy of the Paschal lamb dripped onto the earthenware and returned to it, one must remove its place. It might enter your mind to say that this is referring to cold earthenware. Granted, according to the opinion of Rav, who said the upper one prevails, it is due to this reason that one must remove its place. According to Rav’s view, the gravy goes and heats the earthenware, and then the earthenware heats the gravy, and when the gravy returns to the Paschal lamb, the Paschal lamb becomes roasted from the heat of the earthenware, and God states in the Torah: “Roasted in fire” (Exodus 12:8), and not roasted due to something else.