We have not done any lomdus in the past week, so time to make up for it. My son is learning Bava Kamma in 10th grade and asked the following kashe, which I thought was pretty amazing, but I'm obviously biased : ) He pointed out that R' Elchanan in Koveitz Shiurim Pesachim #17 discusses how to understand the nature of the chiyuvim on shomrim when things go wrong, e.g. an item is lost or stolen -- Is the penalty of payment a result of the Torah imposing a punishment or obligation to make restitution on the shomer, or is the restitution a function of the agreement between the shomer and the mafkid, similar to a contractual obligation?
Without going through R' Elchanan's discussion, one can at least prove that a sho'el is obligated to make restititution by virtue of his agreement with the mafkid. The halacha is that a shoel must always return or pay for the object he borrows, even if the object is lost or destroyed by accident (with the exception of accidental breakage that occurs during normal usage). Yet, Tosfos (Bava Kama 27b d"h u"Shmuel [however, see Ramban, B.M. 82 who disagrees]) holds that a person is not liable for damages (nezek) that occur in a completely accidental mishap -- the Torah never imposes a penalty in cases of oness gamur. Why then is a shoel obligated to pay in a case of oness gamur? The answer must be that even though the Torah does not obligate payment, the shoel assumed that added liability by virtue of agreeing to the terms of shei'lah with the mafkid.
Here's the catch: Bava Kama (4b) quotes a barysa in the name of R' Oshiya that lists 13 avos nezikin, counting among them the four shomrim. Each shomer is counted as a separate av l'nezikin because each has a unique halachic set of rules that govern the shomer's obligation to make restitution for theft or damage. But wait a minute, said my son -- a shoel is unique only in that he pays even in cases of ones, which no other shomeir must do. The reason a sho'el pays in cases of ones has nothing to do with the halachos of nezikin imposed by the Torah -- his obligation stems from his kabalas achrayus, his self-imposed agreement with the mafkid! Why then does this count as a separate category of av nezikin?
I don't feel so bad for being stumped because his rebbe was also stumped. We will see if he comes up with anything.