Shabbat 108 - Writing on fish skin
Rav Huna said, "One can write tefillin on the skin of a kosher bird." Rav asked asked, "What is Rav Huna teaching us? That bird skins qualify as hides? - We know that ! One who bruises a bird on Shabbat is liable, and that can only be because birds have hides, under which the blood collects." His student Abaye told him, "Actually, Rav Huna is teaching us a lot. Since birds skins are perforated with weathers, we might have thought that this makes them invalid for writing a tefillin, and he teaches us otherwise."
Mar bar Ravina asked of Rav Nachman, "Can one write tefillin on the skin of a kosher fish?" Rav Nachman answered, "When Elijah the prophet comes, he will tell us." What kind of answer is this? We see ourselves that fish do have skin. Furthermore, we know the teaching that the skin and the bones of a fish accept ritual impurity of a corpse in the house, so of course they have skin! - He meant, is that skin valid. Does the stench completely depart from it, or perhaps it never does? This is one of the unclear facts that Elijah has the authority to reveal, and that's why we must wait for him.
Art: Jacob Toorenvliet - A Fish Seller