Shabbos, Mishnah 3. In chapter 13, mishnah 3, we learned that one who destroys on Shabbos - one who breaks an object and performs a destructive act - is exempt. Even so, one may not perform a destructive act on Shabbos at the outset. Our mishnah comes to teach us that when it is done for the sake of eating, it is permitted.
Breaking a barrel for the sake of eating:
The mishnah rules that "shover adam es hechavis le'echol heimenah grogaros" - a person may break a barrel in order to eat the dried figs lying inside it. When there is no way to open it other than by breaking it, he is permitted to break it in order to reach what is inside.
One condition is stated regarding this: "uvilvad shelo yiskaven la'asos keli" - when he breaks the barrel or makes a hole in it, he must not intend to fashion a vessel, that is, to improve the barrel by means of a neat hole that will serve for putting things in and taking them out. Such an intention is forbidden because of the melachah of makeh bepatish, whose essence is the finishing touch that completes a vessel.
Making a hole in the stopper - the dispute between Rabbi Yehudah and the Sages:
The mishnah goes on to discuss making a hole in the stopper - the cover that seals the mouth of the barrel. It was customary to place a cover on the barrel, sometimes made of a different material, and to seal it with pitch or wax. When they wanted to remove wine, they would make a small hole in it and tilt the barrel in order to pour through it.
Rabbi Yehudah forbids: one may not make a hole in the stopper of a barrel, since making a proper hole in the cover is considered improving the vessel. In his view, one must remove the entire cover.
And the Sages permit: it is permitted to make the hole in the stopper, because this is not the usual way of making a hole. One of the reasons brought by the commentators: people do not want a hole in the upper part of the vessel, since things are liable to fall through it into the barrel.
"Velo yikvenah mitzidah":
Even the Sages who permit agree that one may not make the hole in the side of the stopper. People do tend to make a hole in the side more often, because there they are not concerned that things will fall inside, as they are with a hole in the upper part. For that reason it is considered fashioning a vessel, and it involves the prohibition of makeh bepatish.
A barrel that was already pierced before Shabbos:
"Ve'im haysah nekuvah, lo yiten aleha sha'avah" - a barrel that was already pierced while it was still day, before Shabbos, one may not place wax on it in order to seal it, "mipnei shehu memare'ach". Smearing the wax is the melachah of memache'ach - smoothing a substance - which is forbidden on Shabbos.
The incident that came before Rabban Yochanan ben Zakkai:
At the end of the mishnah an incident is brought: Rabbi Yehudah said, an incident came before Rabban Yochanan ben Zakkai in Arav (a place name), concerning a man who smeared wax over the hole of a barrel on Shabbos, and he said: "choshshani lo mechatas" - I am concerned that this man may be liable to bring a chatas offering, for it is possible that he unwittingly transgressed a Torah prohibition, and for that he must bring an offering.
In summary: in this mishnah we learned that it is permitted to break a barrel in order to eat the dried figs inside it, provided that one does not intend to fashion a vessel; we saw the dispute between Rabbi Yehudah and the Sages about making a hole in the stopper, and that all agree that one may not make the hole in its side because of makeh bepatish; and finally, the prohibition of placing wax over a hole that already existed before Shabbos because of memare'ach, and the incident that came before Rabban Yochanan ben Zakkai, who said: "choshshani lo mechatas".