The melakha of Lash refers to the act of kneading, forming dough out of flour and water. However, it also includes mixing any liquid with flour to form dough. Even if the liquid used is viscous, like honey or mayonnaise, it is still forbidden to mix it with flour. As long as one binds the flour and forms dough, this is considered Lash. Along the same lines, if one mixes water with dried earth in order to make bricks or spackle to plug holes in a wall, he transgresses Lash.
One may not perform even a single component of the kneading process. Therefore, one may not pour water onto flour. Additionally, after dough has been formed, one may not shape it.
Lash involves forming something new with characteristics that differ from those of its component parts. Flour on its own and water on its own cannot rise and cannot be baked. Only after they are mixed together can they be made into bread and cookies. Similarly, dried earth or spackle on their own and water on its own cannot be used for building. Only after they are mixed together can they be used to form bricks or to plug holes in the wall.
The Torah prohibition on Lash is limited to a thick mixture. Rabbinically, it is also prohibited to make a loose mixture (as will be explained in the next section). However, if the quantity of the solid added to the liquid is so minimal that it dissolves, the liquid retains its form, and no type of mixture results, then there is no prohibition of Lash. Therefore, one may add coffee or sugar to water, as the granules do not form a mass in the water. In addition, one may prepare infant formula by mixing the powder with water since the mixture does not form a mass; rather, the liquid retains its form, and can be drunk from a bottle. Thus there is no problem of Lash. (One should prepare coffee in a kli shlishi to avoid violating the melakha of Bishul; see above, 10:7.)
. Everyone agrees that one who actually forms dough is transgressing a Torah prohibition, but the Tanna’im disagree whether adding water to flour is a Torah prohibition. According to R. Yehuda Ha-Nasi, one who adds water to flour is transgressing a Torah prohibition because he has taken the first step in the process of making dough; according to R. Yossi Be-Rabbi Yehuda, since one has not mixed the water and flour together, he has only transgressed a rabbinic prohibition (Shabbat 18a, 155b). Most poskim, including Rabbeinu Ĥananel, Rif, Rambam, Rosh, Ba’al Ha-Ma’or, Ramban, Or Zaru’a, Roke’aĥ, Rabbeinu Yeruĥam, and Me’iri maintain that the halakha follows R. Yossi Be-Rabbi Yehuda. However, according to Sefer Ha-Teruma, Yere’im, Smag, and Smak, the halakha follows R. Yehuda Ha-Nasi. (Le-khatĥila one should not wash his hands over dirt nor urinate into dirt, because R. Yossi Be-Rabbi Yehuda agrees that those actions are forbidden. In a time of need one may be lenient, as this is a case of a psik reisha de-lo niĥa lei involving a rabbinic prohibition. See MB 321:57.)There are some materials, such as ash, that are incapable of forming dough. If they are mixed with water they do form a type of mass, but it is not stable and it crumbles once it dries. There are three opinions regarding such materials:
- a) One may be more lenient regarding them. He may pour water on the ash, but mixing the two together is rabbinically prohibited (Rambam; Rid; Ritva; and implied by Rif and Rashi).
- b) One must be more stringent with them. Since they cannot really form a mass, one is liable as soon as water is poured on them, as even R. Yossi Be-Rabbi Yehuda would agree (following Abaye in Shabbat 18a; according to BHL 324:3 s.v. “ein” this is the ruling of Tosafot, Rosh, Raavad, Rashba, and Ran; it would also seem to be the opinion of Sefer Ha-Teruma and all who follow R. Yehuda Ha-Nasi’s ruling).
- c) They are the same as any other material that is subject to Lash. The Tanna’im disagree regarding the status of the prohibition of pouring water on them, but agree that mixing them is prohibited by Torah law (following the opinion of R. Yosef; Ĥazon Ish 56:3 maintains that this is the opinion of most of the poskim mentioned in BHL).
. If the mixture is likely to solidify and harden on its own, then when one adds the water he is already transgressing the Torah prohibition of Lash. This is because the essence of the melakha is joining discrete particles into one solid mass. Thus the Sages state that adding water to flax seeds is considered Lash, because the water causes the seeds to secrete a substance that causes them to stick to one another (Zevaĥim 94b). Similarly, it is prohibited by Torah law to put cement, water, and aggregate together to form concrete; although the mixture starts out loose and pourable, the concrete will subsequently solidify on its own (see Ketzot Ha-shulĥan 130:3). Therefore, one may not make gelatin desserts, as explained below in section 7.