By Rabbi Dovid Ostroff
My new tin of Nescafe is sealed with a piece of aluminum foil. Am I permitted to rip it open on Shabbos or is it tearing?
There is a Tosefta in Shabbos 17:9 that says the following: “One is permitted to rip the leather covering a barrel’s opening provided that one does not intend making a spout”. This means that it is permitted to tear the leather, or any covering for that matter, that covers a barrel in order to retrieve the barrel’s contents.
The question is though that we know that tearing paper or leather is forbidden on Shabbos; if done in a constructive manner it involves a Biblical transgression and if done in a destructive manner it involves a rabbinical one.
The Rambam says (10:10) that, one who tears in order to destruct is exempt (from a biblical punishment), but nevertheless is forbidden to do so, so why should it be permitted when fixed to a barrel?
The sefer Sh’visas HaShabbos explains that rags or leather wrapping food or drink become part of the food just as a nutshell is part of the nut, and just as one is permitted to crack open a nut, so too one is permitted to tear the paper or leather encasing food.
The Sh’visas HaShabbos adds that “it becomes like other detached materials where Kore’ah – Tearing is not applicable”. Accordingly, it is permitted to tear the aluminum foil sealing the coffee.  We will explain this in more detail soon
I was presented on Shabbos with a piece of veal tied in a net and did not know how to proceed. Was I permitted to slice the string?
The Shulchan Aruch  says that one is permitted to cut through string tying roast meat or fowl to the spit. The explanation being, as the poskim  put it, that string is not a k’li and when not cut to size or for a constructive purpose it is permitted outright.
Since the string encasing the meat is not cut to size, nor – as far as the string is concerned – is it cut for a constructive purpose (it is immaterial that I have an indirect benefit from cutting the string), it is permitted.
Why then does the Rambam, quoted previously, say that one is forbidden to rip or tear something destructively?
The P’ri Megadim  answered this question saying that a garment, leather or paper are considered keilim and tearing them is akin to destroying a k’li, which is a rabbinical prohibition. Conversely, a piece of string or rope is not considered a k’li and therefore its tearing for no purpose is not an issur at all.
We can now understand the Sh’visas HaShabbos we mentioned above that said that the leather becomes like a detached item i.e. a non-kli and as such Tearing does not apply.
The Shulchan Aruch HaRav  also used the term of cutting a detached item.
Would this imply that I am permitted to tear any cellophane wrapping, whether it be for food, clothing etc?
Correct, because the cellophane wrapper becomes part of the contents and is not looked at as a separate entity, thereby permitting tearing the wrapper to retrieve the contents.
Rav Shlomo Zalman Auerbach Ztz”l  learns from this Sh’visas HaShabbos that one may open any packaging that is not reused after its contents have been emptied (the Rav holds that one need not empty the contents upon opening, nor must the packaging be opened in a destroying manner, but that is a separate issue) because of this very reason. The Rav did not restrict this rule to food wrappings, because this reason applies to clothes and toys as well.
1] If, for example, they were set aside for that use.
 Based on siman 314:10. SS”K 23:17.
 SS”K 25:9.
 Sefer Machze Eliyahu 67:4.
 The source for this issur can be found in Maseches Eiruvin 104a where the kohanim would sprinkle salt to improve the traction on the ramp leading up to the mizbeach after it rained, and the gemora says that it involves an issur of Boneh.
 Rav Sternbuch pointed out that some poskim only permit tearing the paper in a detrimental manner, and therefore, as usual, one should ask one’s rav for the correct halachic procedure.
 Siman 314:9.
 M”A siman 314:14, Shulchan Aruch HaRav 314:2, M”B 314:41.
 Eshel Avraham simon 317:20.
 Simon 314:12.
 Sh’miras Shabbos Kehilchasa vol. III chapter 9 footnote 11. Many of these issues and more can be found in the exceptional sefer on Boneh and Makeh B’patish – Binyan Shabbos, written by Rav Chanan Cohen of Har Nof.