Generate a full detail guide for teachers on 'Differentiated Instruction.' The guide must be clear and practical. It should include: 1. A simple definition of differentiation. 2. A detailed section on the three main areas to differentiate: Content, Process, and Product. For each area, provide multiple practical examples for a sample lesson. 3. A guide to using flexible grouping in the classroom. 4. How to use formative assessments to guide differentiation. Please include a supportive disclaimer.