If you damage an object proactively — even by mistake — it is obvious that you will be liable for that damage like any other mazik. If, however, something gets damaged as meisah machmas melachah — it dies as the result of normal use — you will not be liable for the damage (Shulchan Aruch, Choshen Mishpat 340:1). If you overuse an item, however, then you will be liable if it breaks.
If one of your minor (i.e., under bar or bas mitzvah) children damages an object, since they are not responsible for their damages, according to the letter of the law, you are not responsible for those damages either (Choshen Mishpat 349:3 & 424:8; also see BHI # 167). Nevertheless, it is possible that you will be liable as a shomer in the latter case.
In regard to the apartment itself, real estate (ibid. 301:1), and anything connected to the ground (mechubar l’karka; see ibid. 95:1 with Shach 8) are not included in the halachos of shemirah. Therefore, unless there was a specific condition made, along with a kinyan, obligating you to pay for such damages, you are not liable (see Kovetz Teshuvos 1:216).
In regard to the contents, some Poskim say that a borrower is liable even if no kinyan was made on the borrowed object (Choshen Mishpat 340:4).
We find a case in the Gemara (Bava Kamma 48a; Choshen Mishpat 393:3) in which a woman entered someone’s house to bake, and the homeowner left due to a tznius issue, since her arms would become exposed during baking. While he was out of the house, his animal ate her baked items and was harmed by them. The Gemara rules that since the woman knew that the homeowner would have to leave because of the tznius issue, she became obligated to guard his animal, and she is therefore liable for the damage to the animal.
It would appear from this Gemara that she became responsible for the shemirah even without having made a kinyan. This Gemara therefore seems to contradict the opinion that a person cannot be responsible as a shomer without a kinyan (Ri’az, Kuntres Haraayos, Bava Metzia 81b, Kava D’Kashyasa 24).
There are several answers offered to resolve this difficulty.
1. A kinyan kesef is enough to make a person responsible as a shomer (Machaneh Ephraim, Shomrim 7 and Nesivos 291:12; see also Nesivos 306:1 and 340:8. See, however, Erech Shai and Minchas Pittim 306, Imrei Binah, Halvaah 82).
This woman received something of monetary value from the homeowner when he allowed her to bake in his house, and that served as a kinyan kesef to make her liable as a shomer (Machazeh Avraham (Yafeh), Bava Kamma loc. cit.).
2. The woman purchased the right to use the oven by making a chazakah when she used it, and through a kinyan agav (additive kinyan), she accepted to serve as a shomer (Nesivos 95:1; cf. Mishpetei Hachoshen p. 447).
It would seem that your situation is similar to that of this woman, and the above approaches explaining how she became obligated to guard the homeowner’s animal apply to you as well.
Although some Poskim deliberate whether a shomer becomes responsible for guarding an object with the above-mentioned kinyanim, they maintain that the only kinyan that would suffice to obligate a shomer is when it enters his property — similar to him making a kinyan of hagbahah or meshichah. (See Mishpetei Hachoshen loc. cit.) It is still clear from that Gemara that a shomer could become obligated to guard an item without that type of kinyan. Furthermore, it is possible that he becomes liable due to a kinyan chatzer (see Choshen Mishpat 313).
But the Poskim discuss what type of shomer you are in this case — a shomer chinam, socher or sho’el — and whether there is a difference between objects that you would typically use in the course of borrowing someone’s house and objects that you would not normally use, such as a Chanukah menorah (see Mishpetei Hachoshen p. 423.)
