What is the classic "store / retrieve" pattern used to introduce Solidity state?

A minimal contract with one state variable, a store function that writes it (a paid transaction) and a retrieve view function that reads it (free) — the smallest example of on-chain persistent state.

contract SimpleStorage {
    uint256 favoriteNumber;                 // starts at 0

    function store(uint256 _n) public {     // writes state -> costs gas
        favoriteNumber = _n;
    }

    function retrieve() public view returns (uint256) {  // reads -> free
        return favoriteNumber;
    }
}

It demonstrates the core split: store modifies the blockchain, so it must be a transaction and burns gas; retrieve only reads, so it is a free view call. Adding more work inside store (e.g. favoriteNumber = favoriteNumber + 1;) makes the transaction more computationally expensive and visibly raises its gas cost — a concrete demonstration that gas tracks how much work you do.

Go deeper:

• ethereum.org: anatomy of a smart contract — state variables, functions, and events in one place.