Reusing a Hot Cup on Shabbos

Ask the Rov: Can I reuse my cup with remaining droplets to refill with hot water from the urn on Shabbos?

By Rabbi Chaim Hillel Raskin – Rov of Anash in Petach Tikvah

Hot water in the original utensil in which it was heated (k’li rishon) has the ability to cook other foods, and doing so on Shabbos is an issur d’oraisa. After being transferred into another utensil (k’li sheini), it can no longer cook liquid. Hot water poured (irui) from a k’li rishon is generally treated like a kli rishon.

What about the small drops that remain in a cup after use? Can one use such a cup to fill directly from the urn, or must one seek out a dry k’li sheini to fill from the urn and pour from it into the wet k’li shlishi?

The halacha is that cold liquid is subject to bishul even if previously cooked (see article 681), and the drops usually cool down quickly. Yet, poskim are lenient based on several reasons:1

  1. There is after all a debate amongst poskim whether cold liquid is subject to bishul, and although we rule stringently, it seems that this stringency is miderabanan and not mide’oraisa.2
  2. There is no intention to cook the water, and even though it is inevitable (p’sik reishei), it is not desired (lo nicha lei), since one would prefer that the cold drops not be there altogether.3 Although we do not permit melacha because of p’sik reishei d’lo nicha lei, here it can be combined with other mitigating factors.
  3. The drops are so minimal and have no significance in Although it is prohibited to perform melacha even for a partial amount (chatzi shiur), these drops are so insignificant some hold it does not constitute bishul at all. This can be relied upon here, where there are additional grounds for leniency as well.
  4. The Alter Rebbe rules that the bottom substance in a k’li sheini is more powerful (tata’a gavar) and it can cool off the hot liquid poured into it before it has a chance to cook. It’s unclear if there is a minimum amount or if it applies even to a drop of cold water on bottom.4

In practice, if one shakes the cup to empty it out, one can refill it with hot water and need not worry if some drops remain.5

See Sources (open PDF)

From The Weekly Farbrengen by Merkaz Anash

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