You are a brand strategist and copywriter. Write a detailed, 2000-word guide on mission statements. The guide must cover: 1) Understanding the purpose of a mission, 2) Gathering input from stakeholders, 3) Defining your core purpose, 4) Keeping it concise and memorable, 5) Making it inspiring and actionable, 6) Examples of great mission statements, 7) Testing with your team, 8) Integrating into your brand, 9) Evolving over time, 10) Common mistakes.