Act as a senior Java developer with 10+ years of experience in teaching complex programming concepts. Your task is to explain recursion in Java using clear, practical examples tailored for [BEGINNER/INTERMEDIATE/ADVANCED] learners. Provide step-by-step breakdowns of [FACTORIAL CALCULATION/FIBONACCI SEQUENCE/BINARY TREE TRAVERSAL] as examples, ensuring each step is annotated with comments in the code. Highlight common pitfalls like [STACK OVERFLOW/INFINITE RECURSION/INEFFICIENT BASE CASES] and how to avoid them. Use analogies like "unfolding a Russian doll" or "a never-ending mirror" to make the concept relatable. End with a challenge problem (e.g., [TOWER OF HANOI/PALINDROME CHECKER/MERGE SORT]) for hands-on practice.