The shape of the lesson
By the end of the lesson, learners can
- Digital currency: You may already pay by card or phone, but a digital currency goes further: the money itself is only ever data.
- How a blockchain works: In the lab, look at the chain of blocks and their hashes.
- Why it cannot be altered: The syllabus describes a blockchain as records that cannot be altered.
- Digital currency: only exists electronically.
- Blockchain: a digital ledger, a time-stamped series of records that cannot be altered.
- Block: transactions, timestamp, its own hash, previous block's hash.
- Tampering: hash changes, chain breaks, other copies disagree.
One picture every idea returns to
A digital currency is one that only exists electronically: there are no physical notes or coins. It is used to pay for goods and services and to transfer money between people over the internet. Many digital currencies (cryptocurrencies) track every transaction with a blockchain: a digital ledger, that is a time-stamped series of records that cannot be altered. Each block stores its transactions, a timestamp, its own hash and the hash of the previous block, and copies of the chain are held by every computer in the network.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Money with no coins.. Some money has no notes or coins at all: it exists only as data. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Digital currency | You may already pay by card or phone, but a digital currency goes further: the money itself is only ever data. | Whole class, then pairs |
| 13 to 21 min | Teach and check: How a blockchain works | In the lab, look at the chain of blocks and their hashes. Project tab 4 of the lab (Breaking a blockchain) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Why it cannot be altered | The syllabus describes a blockchain as records that cannot be altered. Model the worked example Tracing a change on the board, one line at a time. | Connect-Extend-Challenge |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 14 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Why it cannot be altered, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Do not describe a digital currency as "money paid with a bank card". | The key point is that the currency only exists electronically. |
| Blockchain does not stop transactions being made, and it is not encryption of the money. | It makes the record of transactions tamper-evident and permanent. |
Digital currency and blockchain: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
State what is meant by a digital currency.
Describe two ways a digital currency can be used.
Describe how blockchain is used to track digital currency transactions.
Explain why a transaction recorded in a blockchain cannot be altered.
Identify two items of data stored in each block of a blockchain, other than the transactions.
A student says: "Blockchain keeps digital currency safe because it encrypts the money." Explain why this is not an accurate description.
Answer key and marking notes
A currency that only exists electronically (it has no physical form).
- To pay for goods and services (online or in shops that accept it).
- To transfer money directly to another person over the internet (from a digital wallet).
- Blockchain is a digital ledger / a time-stamped series of records.
- Each new transaction is checked / verified by computers in the network and added in a new block with a timestamp.
- Each block stores the hash of the previous block, linking the blocks in a chain.
- Every computer in the network holds a copy, so all transactions are recorded permanently and can be traced.
- Altering the data in a block changes that block's hash.
- The next block stores the old hash, so it no longer matches and the chain is broken.
- The altered copy also differs from the copies held by the other computers, so the change is detected / rejected.
- A timestamp.
- The hash of the block / the hash of the previous block (either).
- Blockchain is a ledger / record of transactions; it does not encrypt the currency.
- It keeps the currency trustworthy because the time-stamped records cannot be altered without detection.
6 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What is a digital currency?
- AA currency that only exists electronically
- BAny money that is paid using a bank card
- CBanknotes that have been scanned and stored on a computer
- DA currency that can only be spent in one country
Which list shows what each block in a blockchain stores?
- ATransactions, a timestamp, its own hash and the previous block's hash
- BTransactions and the passwords of everyone involved
- CA timestamp and the IP address of every computer in the network
- DIts own hash and a copy of every other block in the chain
The diagram shows part of a blockchain. A criminal changes Block 2 to read "B pays C 20", so Block 2's hash is recalculated as 5b02. How is the change detected?
- ABlock 3 stores previous hash 81d4, which no longer matches Block 2's hash
- BBlock 2 stores previous hash 3c9e, which no longer matches Block 1's hash
- CBlock 1's hash changes to 5b02 to match Block 2
- DIt is not detected, because Block 2 now stores its own new hash
Who holds a copy of the blockchain for a cryptocurrency?
- AEvery computer (node) in the network
- BOnly the central bank of each country
- COnly the person who made the most recent payment
- DOne secure server owned by the currency's inventor
A criminal edits an old transaction on their own computer's copy of a blockchain and recalculates the hashes of every later block so their copy links up again. Why is the change still rejected?
- ATheir copy no longer matches the copies held by all the other computers in the network
- BThe blockchain is encrypted, so the criminal cannot read any of the transactions
- CTimestamps can only be added by a central bank, which spots the change
- DHashes can never be recalculated once a block has been created
Which is the most accurate description of how blockchain supports a digital currency?
- AIt is a time-stamped ledger of transactions that cannot be altered without detection
- BIt encrypts the money so it cannot be stolen
- CIt stops users from making transactions without a bank's approval
- DIt converts the digital currency into banknotes when needed
Quiz key
A digital currency has no physical form: it exists only as electronic records. Paying by bank card moves ordinary money that also exists as notes and coins. Scanned banknotes are just images, and digital currencies can be sent anywhere in the world over the internet.
Each block holds a group of transactions, a timestamp, its own hash and the hash of the previous block, which links it into the chain. Blocks do not store users' passwords or the IP addresses of the network's computers, and a single block does not hold copies of all the other blocks.
Block 3 still records 81d4 as its previous hash, but Block 2's hash is now 5b02, so the link is broken and Block 3 and everything after it become invalid. Block 1 was not changed, so its hash is still 3c9e and still matches Block 2's previous hash. Changing Block 2 does not alter Block 1. Recalculating Block 2's own hash does not hide the change, because the next block disagrees (and so do the other computers' copies).
Every node in the network holds a copy of the whole chain, so there is no single central copy to attack or alter. Cryptocurrencies are not controlled by a central bank, and the ledger is not kept by one person or one server.
The blockchain is distributed: thousands of other computers hold the original chain, so the altered copy disagrees with all of them and is rejected. A blockchain is a ledger, not encryption of the transactions. There is no central bank stamping blocks, and hashes can be recalculated by anyone; what cannot be done is make every other copy agree.
Blockchain is a digital ledger: a time-stamped series of records, linked by hashes and held by every node, so past transactions cannot be altered unnoticed. It does not encrypt the money, it does not need a bank's approval (cryptocurrencies are not controlled by a central bank), and a digital currency has no banknotes to convert to.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.