Chapter 9
Opening Cans, Bottles and Other Containers on Shabbath and Yom Tov
2.a. Bags, cans or similar, sealed containers (for example sacks of sugar) which it is customary to re-use after they have been opened and their contents removed may not be opened on Shabbath or Yom Tov, unless they are spoiled before or in the course of being opened (as in paragraph 3d below), and their contents emptied into other containers.
2.b. The prohibition applies even if the person who wishes to open the container does not intend to re-use it.
2.c. The reason for the prohibition is that it is the act of opening the container which converts it into a usable article, whereas previously it merely formed the outer skin or shell of whatever was inside it and was not fit for any other purpose.
