Please excuse my weak knowledge in this area. I find the cooking laws of Shabbat quite confusing and I'd like to get a simplified understanding of it, if possible.

Apparently, pouring hot water from a kli rishon (first vessel) into a kli sheini (second vessel) suffices for it not to be considered "cooking".

As I understand, "cooking" is defined as "yad soledet bo" - i.e., if the temperature is so hot that your hand would flinch from the heat, it is considered cooking.

My question on all this. I think most people would agree that even by the time you pour that hot water into the 2nd vessel, it is still yad soledet bo. So, why does the kli sheini technique work?

Also, It seems that the vessel you pour the liquid into should be the biggest factor in temperature change. If one poured the liquid into an ice cold glass beer mug, it would cool much more quickly than if it were poured into a hot aluminum cup that was left on the blech.

If you would, feel free to edit the question if I got any of these concepts way off. My main question is regarding the idea of the kli sheini.

I request that your answer be in "layman's terms". Of course, sourcing is a big help in case I want to research further, but, please try to offer a "simplified" English source. I've looked at the "big" 39 Melachot book in English, and I'm still not quite understanding this.


2 Answers 2


No need to worry about this being a bad question. Its Tosfos's question!

ושמע מינה כלי שני אינו מבשל - תימה מאי שנא כלי שני מכלי ראשון דאי יד סולדת אפי' כלי שני נמי ואי אין יד סולדת אפילו כלי ראשון נמי אינו מבשל ויש לומר לפי שכלי ראשון מתוך שעמד על האור דופנותיו חמין ומחזיק חומו זמן מרובה ולכך נתנו בו שיעור דכל זמן שהיד סולדת בו אסור אבל כלי שני אף על גב דיד סולדת בו מותר שאין דופנותיו חמין והולך ומתקרר:

Essentially Tosfos's answer is that in a Kli Rishon the pot walls are hot so the food stays hot long enough to cook, while in a Kli Sheini the pot walls are cold so the food cools off rapidly.

By Hilchos Shabbos this is pretty easy to understand as the Ohr Sameach explains, because you are only Chayav for a Meleches Machsheves, a Melacha similar to how it was done in the Mishkan. Cooking food in a pot that's rapidly cooling off is not how it was done in the Mishkan. (By Basar Bechalav it's much more complicated. See this Shiur from Rabbi Rudinsky.)

Please note that these laws are very complex and have a lot of exceptions and disagreements.

For example according to the Chayei Adam 59:6 (which Mishna Berura 318:48 agrees with) if the food is hot enough to scald you it will still cook.

Additionally, the Maharshal (Yam Shel Shlomo Chullin 8:71) rules that solids remain a Kli Rishon after being transferred to another dish because the new dish isn't enveloping it and cooling it off the same way a liquid is enveloped by it's surroundings.

This is a very complex question and there is much more to say, but for purposes of brevity we'll leave it here and if you have any additional questions post them in the comments.

  • If you heat up a container of soup in the microwave, IME you'll find it cools off quicker than if you heated it in the oven since the liquid is heating and not the walls of vessel which leaches heat from the food later on.
    – Double AA
    Commented Dec 21, 2017 at 10:22
  • For that reason not all Poskim agree that food from a microwave counts as a Kli Rishon.
    – Eliyahu
    Commented Dec 21, 2017 at 15:07
  • @DoubleAA Practically, how would you run a microwave on Shabbat?
    – DanF
    Commented Dec 21, 2017 at 20:20
  • 2
    @DanF Practically, I wouldn't.
    – Double AA
    Commented Dec 21, 2017 at 20:21

The Chazon Ish explains it like this see here, p 9:

חזון איש אורח חיים סימן נב אות יט

לא מצינו מקור לחלק בין שני לשלישי, ואם דברים מתבשלים בשני,מתבשלים לישי,ואין הדבר תלוי אלא בחם המים,שיהיו חמים שהיד סולדת בהם ...ועל הרוב אין כלי שלישי יד סולדת,לכן הקילו

In my words: Normally, the walls of the vessels cool the contents such that a keli shlishi does not have a temperature above yad soledes bo and therefore nothing cooks in a keli shlishi. (This is taken on as the norm by most poskim.)

The Chazon Ish insists that the “numbering” of the keli is not what matters – the temperature determines whether something cooks or not.

I see that this information is available here, Where does the concept of a Kli Shlishi come from? .

  • See the latest edits. Per DoubleAA's suggestion, I "upgraded" to kli sheini. However, Chazon Ish does state that the number of vessels is not the issue, but the temperature. I'm still not seeing in his explanation (and, apparently, he doesn't follow the opinion) that a kli sheini does work. I would think that the material of the vessel matters the most in cooling down the liquid. If I put the hot water in an ice cold beer mug, it would cool much faster than if I poured it into a hot aluminum cup that was sitting on the blech.
    – DanF
    Commented Dec 20, 2017 at 21:58
  • What does מתבשלים לישי mean? Commented Dec 20, 2017 at 22:30
  • @רבותמחשבות Probably should say מתבשלים בשלישי
    – Zev Spitz
    Commented Dec 20, 2017 at 22:45

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