It's a topic that you'll experience halfway through a graduate-level textbook on databases like Elmasri's Fundamentals of Database Systems. No gentle way to do it, and a student has probably been driven away or not well before they read about it.
blueatlas cited the bit on functional dependencies as "an example of what drives many students out of CS" but I'd say that more than anything it's an example of a tricky topic that ought to be presented by a highly engaging, smart instructor instead of a boring one who may or may not understand the material very well. (in retrospect, my feelings and his aren't mutually exclusive)