Shabbat 48A

אסוקי הבלא ⁠— דזיתים מסקי הבלא, דשומשמין לא מסקי הבלא.

causing heat to rise, i.e., heating food that is not actually insulated in it, but merely resting upon it, the residue of olives causes heat to rise. Therefore, it is prohibited even to place cooked food upon it. However, the residue of sesame does not cause heat to rise to that extent. Therefore, it is permitted to place food upon it.

רבה ורבי זירא איקלעו לבי ריש גלותא. חזיוה לההוא עבדא דאנח כוזא דמיא אפומא דקומקומא. נזהיה רבה. אמר ליה רבי זירא: מאי שנא ממיחם על גבי מיחם? אמר ליה: התם אוקומי קא מוקים, הכא אולודי קא מוליד.

The Talmud relates an anecdote somewhat relevant to the previous discussion: Rabba and R' Zeira happened to come to the house of the Exilarch on Shabbat, and saw this servant who placed a jug [kuza] of cold water on the mouth of a kettle filled with hot water. Rabba rebuked him for having acted contrary to the halakha. R' Zeira said to Rabba: How is this case different from placing an urn on top of another urn, which is permitted on Shabbat? Rabba said to him: There, when he places one urn on top of another urn, he merely preserves the heat in the upper urn; therefore, it is permitted. Here, in the case where he places the jug of cold water on the mouth of a kettle, he is generating heat in the water in the upper vessel; therefore, it is prohibited.

הדר חזייה דפרס דסתודר אפומיה דכובא ואנח נטלא עילויה. נזהיה רבה. אמר ליה רבי זירא: אמאי? אמר ליה: השתא חזית. לסוף חזייה דקא מעצר ליה. אמר ליה: מאי שנא מפרונקא? אמר ליה: התם לא קפיד עילויה, הכא קפיד עילויה.

The Talmud continues: Rabba then saw that same servant spread a kerchief [dastodar] over a vat of water and place a cup used to draw water from the vat, on the kerchief. Once again, Rabba rebuked him for having acted improperly. R' Zeira said to him: Why did you rebuke him? Rabba said to him: Now, see what will happen. Ultimately, he saw that the servant was squeezing out the water that was absorbed by the kerchief, thereby violating a Torah prohibition. Nevertheless, R' Zeira said to him: How is this case different from that of a cloth [parvanka], which one is permitted to spread over a vat even on Shabbat? Rabba said to him: There is a distinction between the two cases: There, in the case of the cloth, he is not particular about it; even if it gets wet, he will not come to squeeze it dry. Here, with regard to the kerchief, he is particular about it, and he will wring it so that it will not remain wet.