Read the passage. The passage defines classification as the act of sorting details by a meaningful principle. It then demonstrates the definition on itself: examples can be classified by source, by function, or by degree of typicality. A bird, a receipt, and a proverb are random as a list, but they become useful if grouped under a clear question, such as evidence from nature, commerce, and culture. What is the main organizational strategy used to clarify classification as a reading tool?