The EVM mental model

The Ethereum Virtual Machine (EVM) is the runtime every node executes. Think of the whole network as a single computer with one giant key-value database called world state. The keys are addresses. Every address is an account with four fields: a nonce, a balance in wei, a codeHash, and a storageRoot pointing at that account's own key-value storage.

There are two kinds of account. An externally owned account (EOA) is controlled by a private key; it has a balance and a nonce but no code. A contract account has code and storage, and no private key: nobody "owns" it, its code decides what it does when called. Deploying a contract is a transaction that creates a new address and puts bytecode there. From then on anyone can send a transaction to that address and the code runs.

world state
├── 0xd8dA…6045 (EOA)       balance 6.7 ETH, nonce 5956, no code
├── 0xA0b8…eB48 (USDC)      balance 0, code 4 KB, storage: balances mapping…
└── 0xB4e1…9Dc (Uniswap)    balance 0, code, storage: reserve0, reserve1…

A transaction is a state transition

Every transaction is a function applied to the world state: state' = apply(state, tx). The EVM loads the target's code, runs it with the transaction's data as input, and every SSTORE instruction modifies that contract's storage. If the code runs out of gas or hits a revert, all changes from that call are discarded and the state is as if it never ran, except the gas is still paid. This all-or-nothing rule is what makes on-chain logic composable: a call either fully happens or fully does not.

Inside a call, the code sees a small context: msg.sender (the address that made this call), msg.value (wei sent with it), msg.data (the calldata), and block.timestamp among others. msg.sender is trustworthy because the network verified the signature before running anything; a contract never has to authenticate the caller itself. That single fact is the basis of every access-control pattern you will write.

The challenge models the machine: a world state, a contract with storage, and a call() that applies a function under msg.sender and rolls back on revert.

Next: your first real contract. A notes registry with create, read, update and delete, in actual Solidity.