# Chain Effort > As later blocks are chained after it, the work to change the block would include redoing all the blocks after it. > > \- Satoshi Nakamoto, Bitcoin Whitepaper Once a block has been created and accepted as valid by other nodes on the network, the network collectively begins trying to build the next block using the hash of this block as part of the new block header. Every block since the Genesis block is built upon a previous block in the chain, so overwriting information stored at a certain point in the chain would require the proof of work on the block containing the information being overwritten, and every block discovered since in order for the network to recognise the new version of the time chain as valid history. Anyone seeking to overwrite a transaction using this method must build the new proof of work chain and outpace the constantly lengthening chain-tip for their blocks to be considered as valid, making it computationally impractical to erase information that has been captured in a block. In this way, information contained in blocks that have an established quantity of proof of work on top of them are considered unchangeable, or immutable. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://hub.bsvblockchain.org/higher-learning/bsv-academy/bsv-theory/proof-of-work/chain-effort.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.