Let’s Talk Size
‘He Took Out Wood to Cook an Egg…’
To be liable for carrying on Shabbos, a person must carry an item of a certain size. (He is not liable, for example, if he carries a speck of dust.) The Gemara lists the minimum size of numerous objects in reference to the laws of carrying.
The Pnei Yehoshua (Shabbos 76b), interestingly, points out that whereas different liquids have different minimum measurements, solids all have the same measurement: k’grogeres (the size of a dried fig). Furthermore, while the measurements of human solids are uniform, the measurements of animal foods are not. What’s the logic?
Halacha leMoshe Mi’Sinai
The Pnei Yehoshua writes, based on Eruvin 4b, that the measurement for solids – k’grogeres – was dictated to Moshe Rabbeinu on Har Sinai. Hence, we cannot apply our own reasoning to differentiate between various solids.
However, Hashem did not give Moshe a specific measurement for liquids or animal foods. Rather, He gave Moshe (and the Sages who would succeed him) the authority to determine the minimum measurements of each item based on his understanding of each item’s importance.
The Gemara rules that if a piece of food smaller than k’grogeres expands to k’grogeres, and a person then carries it outside, he is liable for transgressing meleches hotza’ah. In Meseches Menachos (54a), however, we learn that if a piece of food was smaller than ke’zayis, and then expanded to become ke’zayis, it does not attain ke’zayis status (not in regard to mitzvos nor in regard to aveiros).
The Ramban points out this discrepancy but does not resolve it, confessing that he is unable to find a distinction between hilchos Shabbos and the mitzvos and aveiros mentioned in Menachos.
Two Contrasting Measures
The Ramban’s student, the Rashba, suggests that for other mitzvos and aveiros, we measure size, but for hilchos Shabbos, we measure importance (with size merely being an indicator of importance). Thus, when a small food item expands and becomes the size of a ke’zayis due to air pockets inside it, the actual size of the food has not increased and we should not care that it expanded. Yet, the fact remains that people view a larger piece of food as more important, and thus one is liable for carrying an expanded piece of food on Shabbos.
Perhaps the Ramban thought of this answer but rejected it because the size of k’grogeres is not based on how important a food item is perceived. It is based on a decree given to Moshe from Hashem on Har Sinai. Thus, it should be treated just like the measurement of ke’zayis.