The “nonce” in a bitcoin block is a 32-bit (4-byte) field whose value is adjusted by miners so that the hash of the block is less than or equal to the current target of the network. The other fields can not be modified because they have a defined meaning. Any modification to the block data (such as the nonce) will make the block hash completely different. Since it is impossible to predict which bit combination will give the correct hash, many different nonce values are tried and the hash is recalculated for each value until a hash less than or equal to the current target of the network is found. The required target is also represented by the difficulty, where a higher difficulty represents a lower target. Since this iterative calculation requires time and resources, presenting the block with the correct nonce value is proof of the work.