I held my yearly "Antitrust Economics from 30,000 Feet" class last week. That topic is always a bruiser. I have never found a happy way to communicate the dynamics of supply and demand in markets characterized by differing levels of concentration. I frankly wonder if the idea of setting that topic apart in a separate class meeting is a futile endeavor. I would love to appeal to students' intuitions, but for all my efforts talk of gas stations, wheat farmers, auto makers and Microsoft, I am rewarded mostly with blank stares.
To show this is not off-topic: I deliberately schedule that class for after the close of the add-drop period, for fear of students running for the door!