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See this pebble?" "Yes." "Take it." Eragon did and stared at the unremarkable lump. It was dull black, smooth, and as large as the end of his thumb. There were countless stones like it on the trail. "This is your training." Eragon looked back at him, confused. "I don't understand." "Of course you don't," said Brom impatiently. "That's why I'm teaching you and not the other way around. Now stop talking or we'll never get anywhere.

Christopher Paolini (2014). “The Inheritance Cycle Complete Collection: Eragon, Eldest, Brisingr, Inheritance”, p.187, Knopf Books for Young Readers
See this pebble? Yes. Take it. Eragon did and stared at the unremarkable lump. It was dull black, smooth, and as large as the end of his thumb. There were countless stones like it on the trail. This is your training.