hold the key

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hold the key

To contain or provide the missing element or necessary explanation for something. Scientists are scouring the rainforest for medicinal plants in the hope that one of them holds the key to curing cancer. This passage holds the key to understanding the entire novel.
See also: hold, key
References in classic literature ?
Without a word, he made a grab at Polly's arm, for the hand holding the key was still in her, pocket.