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description: Optimism uses a technology called rollups, Optimism rollups.
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# Optimistic Rollups

The reason they are called rollups is because they bundle or (roll up) the data about hundreds of transactions, any transaction weather it's an NFT mints or token swaps into a single transaction on the Ethereum mainnet (layer 1). When that many transactions are rolled up into a single transaction, the blockchain transaction or "gas" fee required to pay comes down to only the one transaction. That is then distributed to everyone involved.&#x20;

Another reason for the name is because transaction are considered and assumed to be valid until they are proven false. Or like the popular saying, innocent until proven guilty. There is a window of time where transactions that are potentially invalid can be challenged by submitting what is called a "fraud proof" and running the transactions computations with reference to available state data. Optimism will also reimburse the gas that is needed to run the computation of the fraud proof.&#x20;

Optimism uses rollups which we talked about before, it is ideal because when so many transactions are rolled up into a single transaction, the gas fee required to pay comes down to only a single transaction that is distributed across everyone involved. The reason they are called optimism rollups is because transactions are assumed to be valid until proven false, in other words innocent until proven guilty. But there is a time where potential invalid transactions can be challenged by putting in a 'fraud proof' and running the transactions computations with reference with available state data. Optimism will reimburse the gas that is needed to run the fraud proof.&#x20;
