How To: Over-Estimate Everyone

"What's a syllable?" 
      This is the question one of my peers asked me amidst a conversation about iambic pentameter during Poetry Club last Wednesday. My initial reaction was to laugh, I couldn't imagine how someone could have made it so far in their academic career at New Trier without knowing what a syllable is.
      Her question, and more-so my emotional response, struck a cord with me. The fact that it did not occur to me that someone could have so little knowledge about a common word like syllable shocked me, because I learned it early on as a player of the game Charades, and a student of literature and poetry.
        This led me to a new question: What is common knowledge? Is it something like: the grass is green, or if you don't feed your fish, it will die? Or might we wrongly assume that something like: the mitochondria are the powerhouse of the cell, providing the energy a cell needs to live.
      Obviously not all people know about the inner-workings of the cell, but I wonder where we can draw the line between what can be considered common knowledge and what can not. What other things so we assume everyone knows?