Chapter 80:59
Some Activities Forbidden on Shabbos
59. It is forbidden, on Shabbos, to break or cut anything that is not food (1). Food, however, even animal fodder, may be broken or cut (2). For this reason, one may cut a straw to pick one's teeth (3). Similarly, sticks of fragrant wood may be crushed or broken to smell them, even if they are hard (4). They may not, however, be cut to pick one's teeth (5).
FOOTNOTES:
(1) One of the 39 Avos Melachos (prototype prohibited activity - see Halacha Yomi 80:15, note (1)) is "Cutting" ("Mechatech"), which involves intentionally breaking or cutting something to a specific size. This is different to the Av Melacha called "Tearing" ("Koreah") which prohibits tearing or cutting any soft material (for a constructive purpose, but not with intent for a specific size), or separating two items that were sewed or glued together (see Halacha-Yomi 80:50-51).
(2) Anything which is edible for an animal, such as soft pieces of wood, falls under this category of "food", even though it had not been specifically set aside as animal fodder before Shabbos (Mishna Berura 322:9).
(3) Besides the prohibition of "Cutting", there is another prohibition involved here, which forbids turning anything into a useful object on Shabbos ("Tikkun Keli"). Something defined as "food," (in this case, straw) cannot, according to the Gemara, be transformed into a "vessel" ("Keli"), and therefore is not subject to the prohibition.
(4) Since the purpose of the cutting is to increase the fragrance, and not to create a useful object (i.e. a toothpick), it is permissible, even though the wood is hard and thus not suitable as animal food. Pieces of wood which are neither fit for animal food, nor have a fragrance, are considered "Muktza," which means they cannot be moved on Shabbos.
(5) Making a toothpick out of hard wood (which is non-edible for animals) would be a violation of the prohibition of "Cutting" and, according to some, of the prohibition against creating a useful object ("Tikkun Keli"). The Mishna Berura (322:16) writes that many later authorities agree that when cutting hard wood to bring out its fragrance, one must use one's hands, not a knife, due to a concern that if one used a knife, one might decide to form a useful object, like a toothpick.
