" To keep up interest in a subject, a teenager has to enjoy working in it. If the teacher makes the task of learning excessively difficult, the student will feel too frustrated and anxious to really get into it and enjoy it for its own sake. If the teacher makes learning too easy, the student will get bored and lose interest. The teacher has the difficult task of finding the right balance between the challenges he or she gives and the students' skills, so that enjoyment and the desire to learn more result. "

― Mihaly Csikszentmihalyi , Creativity: Flow and the Psychology of Discovery and Invention