Understanding how to use testnets is essential for developing smart contracts on Ethereum [https://karl.tech/ethereum-in-101-words/]. Here is a basic introduction to what testnets are and how to use them. What is a testnet? Testnets simulate the Ethereum network and EVM [http://ethereum.stackexchange.com/questions/142/what-was-the-reason-to-invent-the-evm] . They