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There is a concept of ein bishul achar bishul (you can't cook a food more once its cooked already) . This concept applies to dry foods according to all opinions and one would be able to warm such food up without worrying about cooking it. However, with regard to liquids ashkanazim are machmir and hold that there is more cooking that could be done to a liquid.

Many people are aware of this and will not warm up a liquidy food such as a saucy chicken dish on shabbas day. My question is why can't one just pour hot water from an urn (kli rishon) into a kli sheni (empty aluminium pan) and then take the saucy chicken (which is in a smaller aluminum pan) and place it in the kli sheni to warm it up. People do this every shabbas when they pour cold milk into their coffee. Why do people refrain from warming up cold (already cooked) sauce in a kli sheni?

Any time I see this halacha mentioned in a sefer or on a halacha sheet for the rules of warming up, they unequivocally say that warming liquidy foods is assur, but why wouldnt they give such advice?

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    Would that really be practical at warming up a large tray of chicken? – Double AA Dec 18 '17 at 4:46
  • Ofcourse,kli sheni water is still hot enoufh to warm stuff up – sam Dec 18 '17 at 4:47
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What you describe is indeed a common way of re-heating a baby bottle on Shabbat. One places the baby bottle full of cold milk in a cup of hot water until it warms up (although one does not submerges the bottle completely because of hatmana).

This might not practically apply to reheating dishes as it would likely require large amounts of hot water.

For the reasons this is not mentioned more often, it could be that publishers of summary halachot are afraid that people will start cooking for real, e.g., putting a plate with warm water on fire left on. The boundaries between permitted/forbidden are delicate and any advice can easily be misinterpreted. Similarly many (but not all) permit reheating a dish with a bit of sauce, i.e., if the goal of reheating is to reheat the dish and not the sauce, and if the sauce is a minor part of the dish (see e.g., here). But there again this can lead to mistakes and is best discussed with a rav using the specifics of the dish.

  • This isnt a complicated halacha,its knowing the diff netween kli rishon and kli sheni,thats it – sam Dec 18 '17 at 17:16

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