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There are 3 kinds of actions available for you on the Ethereum blockchain: transfer funds from one address to another, a smart contract is created, and there is Ether (ETH) - a fuel that helps to run the blockchain. Gas is used to pay the miners (who secure transactions on the blockchain) for their efforts. The higher the price — the higher priority and favour for the transaction to be completed. Gas limits also protect the network from frauds, when scammers set up unlimited repetitions of transactions and get benefit of it.