One of the questions that comes up most regularly in English class is: "What's a theme?"
This is not an easy question to answer. And often, in our desire to answer quickly and confidently, we respond with a definition that we soon regret. Either because we have rushed to over-simplify our explanation before the student's attention span runs out and they become distracted. Or, we give an explanation so complex and sophisticated that the precipitant result is incomprehension and the student will know better than to ask questions again in future.
I've found the above video very helpful when exploring the concept of 'Theme' with Year 7 or Year 8 classes. It uses a lot of pop culture examples and references to novels that we either study in our curricula or as part of wider reading programs.