# Immutable Work

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*Once the CPU effort has been expended to make it satisfy the proof-of-work, the block cannot be changed without redoing the work.*

Satoshi Nakamoto, Bitcoin Whitepaper

The nature of Proof-of-work is that it **can only be attempted upon a fixed block of information**. In this way, capable nodes will always ensure that power expended performing Proof-of-work is used on blocks that are fully complete and do not waste energy. Once discovered, **a block with a valid Proof-of-work solution cannot be modified in any way without changing its message hash and invalidating the work**.

Nodes who waste hashpower will find that the owners of the hashpower soon migrate to other more capable nodes, reducing their ability to generate blocks and cutting their revenue generating ability. This **incentivises all nodes to act with honesty and to create blocks that are both valid and economical**.


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