well, nothing tricky, really, but I think what Joel said about pointers ( http://www.joelonsoftware.com/articles/GuerrillaInterviewing... ) applies to callbacks (although the starting base would be the 20 students left behind after the pointers apocalypse)
"In first year computer science classes, there are always about 200 kids at the beginning of the semester, all of whom wrote complex adventure games in BASIC for their PCs when they were 4 years old. They are having a good ol’ time learning C or Pascal in college, until one day the professor introduces pointers, and suddenly, they don’t get it. They just don’t understand anything any more. 90% of the class goes off and becomes Political Science majors, then they tell their friends that there weren’t enough good looking members of the appropriate sex in their CompSci classes, that’s why they switched. For some reason most people seem to be born without the part of the brain that understands pointers"
