The Proof-of-Work symbolizes a measure of the resources and time required to complete a task. In the context of crypto-currency, a Proof-of-Work is required when adding new blocks to the Blockchain. The proof-of-work is asymmetric, which means that it must be difficult to provide for the applicant (in this case the miner) but very simple to verify for third parties (the full nodes). On Ethereum, proof of work is provided by the presentation of a valid nonce. This iterative work consists in incrementing the nonce by adding 1 to each trial, until finding a value allowing the hash of the block to be lower than the current target. There is also a mixhash which can also be incremented if all the possibilities of the nonce have been tried without success. In addition to this, the timestamp naturally changes every second, which has a direct consequence on the hash. The nodes will only have to check by generating the hash of the block broadcast (including the nonce and mixhash) on the network to validate or invalidate the block, without having to do the work necessary to discover these parameters again.