Final answer:
The fire was left by Victor in Chapter 11. When the monster saw the fire, he approached it out of curiosity.
Step-by-step explanation:
The fire was left by Victor in Chapter 11 of Shelley's Frankenstein. Victor, after being rejected by his creation, sought solace in the mountains. He built a fire to warm himself, but when the monster saw the fire, he approached it out of curiosity and discovered the concept of fire.
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