What is a smart contract in plain English?
A smart contract is a piece of code with its own stored data, living at a fixed address on a blockchain, that does the same thing for anyone who uses it. Ethereum.org compares it to a vending machine: the right inputs always produce the same output, and the rules are built in before anyone walks up.
Picture a fictional community energy co-op, Brookfield Solar, whose treasurer, Ana, has been asked to look into smart contracts. Her notes from a meeting say: 'smart contract = legally binding?? could auto pay members their share of panel income. who updates the electricity price? what if there's a bug, can we change it? someone said it can't be hacked because blockchain'. Three of those four ideas need correcting, and this lesson does that.
How does a smart contract run on a blockchain?
A smart contract runs when a transaction calls one of its functions, and every computer in the network that checks the transaction runs the same code and reaches the same result. That is why the outcome can be trusted without a middleman: anyone can verify it on the public record.
1. Written
A developer writes the rules in a smart contract language such as Solidity, for example 'if a member's share is due, send it to their address'.
2. Deployed
The compiled code is published to the blockchain at an address. On Ethereum this costs a fee, called gas, and ethereum.org notes contracts cannot be deleted by default.
3. Called
A person or another contract sends a transaction to that address. Ethereum.org describes public contracts as open APIs that other contracts can use, which is called composability.
4. Executed and recorded
The network runs the code deterministically, updates the contract's stored data, and the result becomes part of the shared record.
What are smart contracts used for?
Smart contracts are used wherever rules can be written precisely and the parties want an outcome no single organisation controls. Ethereum.org lists uses such as stablecoins, tokens, unique digital items (NFTs), automatic currency exchanges, games and insurance that pays out when a defined condition is met. In each case the value comes from shared, checkable rules, not from the word 'smart'.
What are the limits of smart contracts?
The limits of smart contracts come from the same properties that make them useful: they only know what is on the blockchain, they are hard to change, and they do exactly what the code says, even when that is a mistake.
Code is not law
A smart contract is a program, not automatically a legal agreement. Whether it creates legal obligations depends on the law and any surrounding agreement, so take legal advice for anything binding.
The oracle problem
Contracts cannot see real-world data such as an electricity price on their own. Oracles feed it in, and ethereum.org notes a contract is only as reliable as the data it receives.
Immutability
Deployed code usually cannot be edited. Bugs stay unless the design included a way to upgrade, which adds its own trust questions.
Size and cost
Ethereum.org notes a contract has a 24KB size limit, and every action costs fees, so complex logic is expensive.
What does a weak and an improved explanation look like?
Weak summary for Ana's committee: 'A smart contract is a legally binding agreement on the blockchain that can't be hacked and will pay members automatically.' Each claim misleads. It may not be legally binding, contracts are hacked when their code has flaws, and paying from panel income needs the income figure from outside the chain.
Improved summary: 'A smart contract is a public program on a blockchain that could hold funds and pay members by fixed rules. It would need an oracle or a trusted person to report each month's income, so we would still depend on that data. Once deployed, fixing mistakes is hard, so the code would need careful review. It does not replace our member agreement, and we would take legal advice. A shared spreadsheet and bank transfers may do the job more simply.' The committee can now decide on facts.
How do you check a claim about smart contracts?
You check a claim about smart contracts by asking four questions: what data does it need from outside the chain, who can change the code, what happens if there is a bug, and does a simpler tool do the job. If a pitch cannot answer all four, treat it with caution.
What are the common misconceptions?
The most common misconception is that 'on the blockchain' means 'cannot go wrong', when a bug in the code can lose funds permanently. Others: assuming a smart contract is a legal contract, forgetting the oracle, treating every use case as better on a blockchain, and confusing explaining a technology with recommending a token. Nothing in this lesson is investment advice.
Where does this lesson sit in Tosheo.ai?
This lesson sits in the Blockchain category and the blockchain course, which gives working professionals the foundations to judge blockchain proposals. In Tosheo.ai the practice task asks you to correct a misleading pitch like Ana's notes, marked against a rubric. Learners who want to write contracts can continue to smart contract security and the optional Code track. It is a professional foundation, not a developer bootcamp, and it never gives investment advice.
Sources and scope
ethereum.org, introduction to smart contracts (checked 19 October 2026) ↗
ethereum.org, smart contracts: what they are and their uses (checked 19 October 2026) ↗