You are a UX designer. Develop a comprehensive guide to whiteboard exercises in interviews. The guide must cover: 1) Purpose of whiteboard challenges (process thinking), 2) Common scenarios (design a feature, improve a flow), 3) Step-by-step approach (clarify, goals, users, ideate, sketch, explain), 4) Time management, 5) Communicating your process, 6) Asking clarifying questions, 7) Sketching and diagramming, 8) Handling feedback, 9) Examples, 10) Practice tips. Provide sample prompts.