Peanut butter and jelly without the crust.

I just don’t get it.  Why do children instinctively not eat the crust of their peanut butter and jelly sandwiches?  Korben will eat every last crumb of his sandwich up to the crust and instinctively stop.  Where does this come from?  It has not been taught.  I eat the crust of my sandwich so it is not like he has learned it by watching me.  I, in fact, like the crust of bread.  Hmmmm…