No coding experience needed — read this like a recipe. This is the classic first contract (Solidity, Ethereum’s main language), storing one number anyone can read but only the owner can change.

contract SimpleStorage {
    uint256 private favoriteNumber;   // the stored value
    address public owner;             // who deployed it

constructor() { owner = msg.sender; }

function store(uint256 _n) public { require(msg.sender == owner, “not owner”); favoriteNumber = _n; // state change = costs gas }

function retrieve() public view returns (uint256) { return favoriteNumber; // read-only = free } } “`

Line by line

– State variables (favoriteNumber, owner) live on-chain permanently. Writing them costs gas; the chain stores them forever. – constructor runs once at deployment, recording the deployer as owner. – store changes state — costs gas, needs a signed transaction, and rejects non-owners. – retrieve is view (read-only): free, instant, no transaction needed.

The three lessons hidden here

1. Reads are free, writes cost. Browsing contracts costs nothing; changing anything costs gas — the economic spam filter. 2. Permissions are just require statements. “Only owner” is one line of code — and forgetting it has caused hundred-million-dollar hacks. 3. Everything is visible. The code (if verified), every call, every stored value — all public on an explorer. Privacy must be designed in; it is never default.

You can now read the skeleton of every contract: state, permissions, and
> which functions cost money. That’s 80% of contract literacy.

Next lesson: tokens as contracts — ERC-20 anatomy.