Opening a can on Shabbos involves a host of halachic issues, some of which are subject to serious debate amongst contemporary poskim. The issues include the risk of transgressing Shabbos isurim, prohibitions, in any one of the following ways: a. fashioning or building a new vessel, b. demolishing an existing vessel, c. the possibility of mechatech, cutting to size, if opening a can on the perforations, and d. making a pesach yofe, a nice opening, which is a derivative of makeh be’patish, completing the final stage of a vessel. For all these reasons and others, it is a good idea to open cans before Shabbos. In the event that there is a great need to open one on Shabbos, there are those poskim who allow it, although some poskim maintain that it is always forbidden, even under extenuating circumstances. One creative approach—though not that easy—to satisfy most of the concerns if one absolutely must open a can on Shabbos, is to open it from both the top and the bottom simultaneously with two can openers, and to remove the entire top and bottom, but not to open the can on the perforations.