Mento is an automated market maker (AMM) type of decentralized exchange that’s native to the Celo platform. It’s responsible for helping the Celo Dollar (cUSD) maintain its peg to the USD. When demand for the stablecoin increases the Celo protocol mints cUSD and ‘sells’ them for CELO (Celo’s native asset) via Mento. The CELO assets are used as cUSD collateral and are stored in the Celo reserve. This all happens on-chain. If demand for cUSD decreases then the reverse process happens. Mento arbitrage opportunities arise in cases where there’s cUSD/USD depeg. This incentive for traders should help restore the cUSD/USD peg. More info here: https://medium.com/celoorg/zooming-in-on-the-celo-expansion-contraction-mechanism-446ca7abe4f