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Competition for a financial reward is also what keeps Bitcoin’s blockchain secure. If any ill-motivated actors wanted to change Bitcoin’s blockchain, they would need to compete with all the other miners distributed globally who have in total invested hundreds of millions of dollars into the machinery necessary to perform PoW. The miners compete by searching for the solution to a cryptographic puzzle that will allow them to add a block of transactions to Bitcoin’s blockchain.
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Chris Burniske (Cryptoassets: The Innovative Investor's Guide to Bitcoin and Beyond)