When someone gives fruit, wine or oil to his friend to watch for him, the shomer – the person who is guarding the merchandise – should not mix it with his own; he should return the exact item that he received. The Mishnayot on today’s daf discuss a person who was supposed to guard and return the items, but instead mixed them with his own. When he does return them, how much does he have to give back? The Mishnayot teach that there is a certain amount of shrinkage or loss that is anticipated in different types of crops, and the shomer will have to give back just the amount that we anticipate would have remained from what he was originally given.
As an example, the second Mishna teaches that when wine was given to be watched, a certain amount must be subtracted for sediment in the wine that settled and also for the amount that we assumed was absorbed by the vessel in which the wine was stored. While the Mishna appears to present a difference of opinion between the Tanna Kamma and Rabbi Yehuda about the amount that is to be subtracted – the Tanna Kamma rules that it is one-sixth of the volume of the wine, while Rabbi Yehuda rules that it is one-fifth of the wine’s volume – the Gemara explains that the vessels that were used to store the wine were different in different communities. In Rabbi Yehuda’s community where the wine was stored in plain earthen vessels, the container absorbed more wine, while in communities where the vessel was covered with wax there was less absorption.
According to the Rambam in his commentary to the Mishna, the amount of loss that the Mishna describes in all of these cases applies in Israel during Mishnaic times, but these measurements will differ depending on weather and other conditions and must be adjusted for every time and place.