#CreatorPad Users broadcast cryptographically signed messages to the network using bitcoin cryptocurrency wallet software. These messages are proposed transactions, changes to be made to the ledger. Each node has a copy of the entire transaction history of the ledger. If a transaction violates the rules of the bitcoin protocol, it is ignored, as transactions only occur when the entire network reaches a consensus for them to take place. This "full network consensus" is achieved when each node in the network verifies the results of a proof-of-work operation called mining. Mining groups transactions into blocks and produces a hash code that follows the rules of the bitcoin protocol. Creating this hash requires costly energy, but a node in the network can verify that the hash is valid using very little energy. If a miner proposes a block to the network, and its hash is valid, the block and its ledger changes are added to the blockchain, and the network moves on to yet unprocessed transactions.