After discussion of the tefillin shel yad - including which arm - the Gemara turns to what is received as an impossible case: for a person with two heads, which one gets the tefillin? But if a two-headed tefillin-wearer was an impossibility, what about a first-born male infant who needed a pidyon ha-ben? Would that be 5 sela'im or 10? Will such a baby make it to the 30-day mark when a pidyon ha-ben takes place? And if not, would he still need the redemption of the ceremony with the kohen? Also, moving on to tzitzit: fringes on a minimum of the 4 corners of a garment (is that one mitzvah or 4?). What about garments with other number of corners? What about wearing tzitzit at night?