--- ### Payback or Revert So, how does a `flash loan` allow the average user to take advantage of larger scale financial opportunities? This is a really powerful function of the blockchain and smart contract ecosystem. In contemporary finance typically collateral is required to take a loan, some assurance must be made that a loan will be repaid. In Web3 a protocol is able to programmatically assure a loan is repaid. **_How does this work?_** A smart contract protocol assures that a `flash loan` is repaid effectively by containing logic within its loan functionality that requires the transferred balance be restored to the protocol within the _same transaction_ as it's borrowed. If these checks don't pass, the transaction will revert, back to its initial state - as though the loan never took place. The code for a `flash loan` may be as minimal as: ```js uint256 startingBalance = IERC20(token).balanceOf(address(this)); assetToken.transferUnderlyingTo(receiverAddress, amount); // callback function here uint256 endingBalance = token.balanceOf(address(this)); if (endingBalance < startingBalance + fee) { revert(); } ``` Effectively, a user taking a `flash loan` is able to do anything they want between the `transferUnderlyingTo` and the conditional check at the end of this function. This is only possible because if that check on the `endingBalance` doesn't pass, the entire transaction (and anything that was done with the loan) will revert! ::image{src='/security-section-6/5-pay-back-or-revert/pay-back-or-revert1.png' style='width: 100%; height: auto;'} It's easy to see what opportunities a system like `flash loans` enables for the average user. No longer will these advantages be available only to whales! See you in the next lesson.