Many items naturally exist as organic wholes, but people have learned to break them down and grind them in order to create new and wonderful things. By grinding wheat and other grains, we make flour, which in turn can be used to bake bread, cake, pasta, and the like. Cayenne pepper is made by grinding hot peppers, and coffee is made by grinding coffee beans. By grinding certain healing plants, medicines are made; by grinding other plants, dyes are prepared. Since grinding is a creative activity, it is included in the melakhot prohibited on Shabbat. Grinding metal and crumbling a block of dirt or clay are forbidden as well.
Generally, grinding an object turns it into flour or powder, but the prohibition of Toĥen can also apply to breaking something down into small pieces. For example, one who chops trees into small bits of wood so that they will easily catch fire transgresses the prohibition of Toĥen (Shabbat 74b).
If one wishes to grind hot peppers or the like to prepare food on Shabbat, he may do so as long as he incorporates two shinuyim (changes) into the process. Traditionally, grinding required two utensils: the mortar upon which the material to be ground was placed, and the pestle that was used to crush and grind this material. Grinding with these two utensils is prohibited by Torah law. Grinding using one of them is rabbinically prohibited. For Shabbat use, however, one may grind food as long as two shinuyim are made. For example, the handle of a knife can be used instead of a pestle, and a plate can be used instead of a mortar (SA 321:7). Although the Sages generally forbid food preparation involving melakha even when it is performed with two shinuyim, here they permit it since the action is considered derekh akhila rather than derekh melakha.
The prohibition of Toĥen applies to natural items like plants, fruits, and metals, but does not apply to foods that have already been ground and then recombined. Therefore, bread, matza, cookies, chocolate, or hardened sugar may be made into crumbs (Rema 321:12). Similarly, one may crumble tobacco powder that has hardened. Furthermore, a sick person who has difficulty swallowing pills may crush them, since their components were already ground before they were made into a pill. In such a case there is no problem of Toĥen (SSK 33:4). Some maintain that one may crush the pills only for immediate use, and le-khatĥila this opinion should be followed (Ĥayei Adam 17:4). In any case, even when one may grind or crush something, one may not use utensils explicitly designed for these purposes, like a grater.
One who scrapes off clay or mud that has hardened naturally from a surface and crumbles the dirt in order to use it transgresses a Torah prohibition. If he does not need the dirt, he is transgressing rabbinically. Therefore, if mud or clay has dried on clothes or shoes, it may not be removed if doing so will definitely make it crumble. Only if it is uncertain whether the mud will crumble may it be removed. Even if it is clear that it will crumble, if necessary it may be removed with a shinui. For example, one may remove the mud on his clothing by using the back of his hand, or he may use one shoe to rub the mud off of the other one.