A mind mapping exercise is a quiet individual brainstorm session, where you write or draw every thought in your mind with no specific order or formatting. The thoughts could be connected, or could be not. The key is to lay out all the thoughts as a pathway or map to possible solutions.
Example:
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References:
https://library.gv.com/the-product-design-sprint-diverge-day-2-c7a5df8e7cd0