Block #1Not valid
Hash
A hands-on crypto course
Break the Chain
Learn how crypto actually works by using it. Break a blockchain, forge a signature, call a smart contract, trade against a liquidity pool and get liquidated, all safely in your browser.
The course
Go in order or jump to what you're curious about. Your progress is saved in this browser.
Module 1
Blockchain basics
Seven hands-on steps, from a single fingerprint to a whole network agreeing on one history.
Step 1 · Fingerprints
Every piece of data has a fingerprint
A hash function turns any text into a 64-character code called a hash. The same text always gives the same hash.
Change anything, even a single letter, and the hash changes completely. There is no way to make a small edit and keep the old hash.
Try it: edit the message, or press the button to change one character.
Why it matters
If data has been changed, its fingerprint gives it away.
Fingerprint (SHA-256 hash)
Each colored square shows two characters of the hash, so changes are easy to see.
Step 2 · Mining
A block only counts if its hash starts with zeros
A block bundles some data with an extra number called the nonce. The network sets a rule, for example that the block's hash must start with 000.
Nobody can predict which nonce will work, so the only way is to keep guessing. That guessing is called mining. Each extra zero makes it about 16 times harder.
Try it: press Mine. Then edit the data and watch the block stop being valid.
Why it matters
A valid block proves that someone spent real effort to make it.
Rule: hash must start with
Press Mine to start guessing.
Step 3 · The chain
Each block carries the hash of the block before it
That link is what makes it a chain. Block #2 includes block #1's hash. So if block #1 changes, block #2 no longer matches, and neither does anything after it.
Try it: change an amount in block #1, or let the button do it. Then try to cover your tracks by re-mining.
Why it matters
To change old history, you have to redo all the work that came after it.
Step 4 · The network
So what stops a cheater from re-mining everything?
You just saw that a cheater can repair a broken chain by re-mining it. What stops them is everyone else.
Thousands of computers, called nodes, each keep their own copy of the chain and compare. A copy that doesn't match the others is rejected, even if every block in it is valid.
Try it: let Omer cheat, then see what it would take for him to win.
Why it matters
No single person controls the record. Rewriting history means out-working most of the network.
Step 5 · Wallets and signatures
Your wallet is a pair of keys
A wallet doesn't hold coins. The coins are recorded on the chain. What a wallet holds is a private key, a secret number only you know, and a public key that everyone can see. Your address is made from the public key.
To spend coins, you sign the transaction with your private key. Anyone can check the signature against your public key, but nobody can produce it without your private key. If any detail of the transaction changes, the signature stops matching.
Try it: sign a payment from Maya, then tamper with it or try to forge one.
Why it matters
Owning crypto means holding the private key. Lose it and the coins are gone for good. Share it and they belong to whoever has it.
Maya's walletReal keys, made in your browser
Address (public)
Creating keys…
Private key (secret)
Hidden
Transaction sent to the network
Signature
Bitcoin and Ethereum use a curve called secp256k1. This demo uses P-256, which works the same way and is built into your browser.
Step 6 · Fees
Block space is limited, so people bid for it
Each block only has room for so many transactions. Waiting transactions sit in a queue called the mempool. Miners and validators fill each block with the best-paying transactions first, because they keep the fees.
When the network is quiet, a tiny fee gets you in. When everyone rushes to trade at once, fees shoot up.
Try it: set your fee and produce blocks until your transaction gets in. Then switch to a busy network and try again.
Why it matters
Nobody sets fees. They come from an auction for limited space, which is why they spike during market panics.
Network
Waiting in the mempool
Last block (5 spots)
Step 7 · Proof of work vs proof of stake
Who gets to add the next block?
Every blockchain needs a fair way to pick who adds the next block, one that's expensive to cheat. There are two main approaches.
Proof of work (Bitcoin, Dogecoin): the winner is whoever finds a valid nonce first, like you did in step 2. Your chance equals your share of the network's computing power, and every guess costs electricity.
Proof of stake (Ethereum, Solana, Cardano): validators lock up coins as a deposit, called a stake. The network picks the next one at random, weighted by how much each has staked. Cheat, and part of your stake is destroyed. That's called slashing.
Try it: pick a method and your share, then run 100 blocks.
Why it matters
Both methods make cheating cost more than it could earn. Proof of stake does it with locked-up money instead of electricity, which is why Ethereum cut its energy use by over 99% when it switched in 2022.
Method
Blocks you madeBlocks everyone else made
Module 2
The major coins
Thousands of cryptocurrencies exist, but a handful hold most of the value. They aren't all trying to do the same job.
1 · Coin or token?
Some run their own chain, most don't
A coin is the native money of its own blockchain. You need it to pay that chain's fees.
A token lives on somebody else's blockchain as a smart contract. Most of the thousands of assets out there, including the big dollar stablecoins, are tokens.
Why it matters
When you hold a token, you depend on two things working: the chain it lives on and the contract that defines it.
Coin
Has its own blockchain, its own nodes and its own rules for making blocks.
Bitcoin, Ether, SOL, XRP, BNB, DogecoinToken
A balance table inside a smart contract on another chain. Anyone can create one.
USDT, USDC, UNI, LINK, most memecoinsModule 4 shows you the actual code behind a token.
2 · Profiles
How each one works
Open at least three to complete this step.
3 · Side by side
The same questions, seven different answers
4 · Match the job
Which one fits?
Each coin is built for different trade-offs. Pick the best fit for each situation.
Why it matters
"Which crypto is best?" has no single answer. It depends on what you need it to do.
Module 3
Crypto markets
How the top coins did over the past year, and how a price actually gets made when someone presses Buy.
1 · Dashboard
The biggest coins by market value
What $100 became over 12 months
Typical monthly move is how much the price usually changes from one month to the next (the standard deviation of monthly returns). Worst drop is the biggest fall from a previous high, using month-end prices. Market data provided by CoinGecko. Live prices from Coinbase.
2 · Order books
Big orders move the price
On an exchange like Coinbase or Binance, the price is just the last deal between a buyer and a seller. Sellers post asks, the lowest price they'll accept. Buyers post bids. The gap between the best ask and the best bid is the spread.
A market order takes whatever is on offer, cheapest first. A small order fills at the best price. A big one eats through several levels, so you pay more on average. That extra cost is called slippage.
Try it: size a Bitcoin buy order, then place it and see where the price ends up.
Why it matters
Liquidity, meaning how much is on offer near the current price, decides how far a trade moves the market. Thinly traded coins swing harder.
3 · What moves prices
Six forces behind the chart
Supply schedules
Bitcoin's new supply halves about every four years. Fewer new coins coming to market has come before big rallies in the past, though past cycles guarantee nothing.
Big buyers and sellers
Since January 2024, US spot Bitcoin ETFs let people buy Bitcoin through an ordinary brokerage account. Money flowing in and out of these funds now moves the price.
Interest rates
When borrowing is cheap, investors take more risk and crypto tends to rise. When rates climb, crypto has often fallen along with tech stocks.
Leverage and liquidations
Many traders borrow to bet bigger. A sharp drop forces automatic selling of their positions, which pushes prices down further.
Regulation and news
Approvals, bans, lawsuits and hacks hit prices within minutes. Crypto trades 24 hours a day, every day, so big moves often land at night or on weekends.
Market value isn't money in
Market value is price times supply. If a coin's last trade jumps 10%, its market value jumps 10%, even if very little money actually changed hands.
Module 4
Smart contracts
Programs that live on a blockchain and move money by themselves. Once deployed, nobody can quietly change or stop them, not even the person who wrote them.
1 · An escrow deal
Two strangers, one bike, no middleman
Maya wants to buy Omer's used bike online. She doesn't want to pay before it arrives, and Omer doesn't want to ship before he's paid. Normally they'd need someone both of them trust.
Instead they use a smart contract. Maya pays into it, and the code decides when the money goes to Omer or back to Maya. Every call costs a small fee called gas, even calls that fail.
Try it: run the deal. Then reset and see what happens if Omer tries to grab the money, or if the bike never arrives.
Why it matters
The rules are enforced by code everyone can read, instead of by a company or a court. That's powerful and risky: if the code has a bug, the bug is enforced too.
BikeEscrow contract
Waiting for paymentThe code
Simplified Solidity. Highlighted lines ran on the last call.Transaction history
2 · Tokens
A token is a contract with a balance table
USDT, USDC and thousands of other tokens aren't separate blockchains. Each one is a smart contract whose main job is to keep a table of who owns how much.
Sending a token means calling the contract's transfer function. It checks you have enough, then moves the numbers.
Try it: send Demo Dollars between people. Then try to send more than someone has.
Why it matters
When people say a token is on Ethereum, they mean its balance table lives inside a contract on Ethereum.
Demo Dollar (DUSD) balances
3 · Limits
What smart contracts can't do
See the outside world
A contract only knows what's on its own chain. To use a price or a sports score, it relies on an oracle, a service that posts outside data on-chain. If the oracle is wrong, the contract believes it anyway.
Fix their own bugs
In 2016, an attacker used a bug in a contract called The DAO to drain about $60 million worth of Ether. Ethereum reversed it with a controversial fork. That almost never happens; usually stolen funds are simply gone.
Keep secrets
Anyone can read a contract's code and every call ever made to it. That's great for trust and bad for privacy.
Run for free
Every step costs gas. On a busy network, a complicated contract call can cost more in fees than the money it moves.
Change their minds
Once deployed, a contract does exactly what its code says. Upgrades are only possible if the authors built in a way to do them, which means trusting the authors again.
Undo a mistake
Send tokens to the wrong address or approve a malicious contract, and there's nobody to call. Transactions are final.
Module 5
DeFi
Decentralized finance rebuilds exchanges, loans and savings as smart contracts. Anyone can use them, at any hour, without an account or a credit check.
1 · Swap pools
An exchange with no order book
A decentralized exchange like Uniswap doesn't match buyers with sellers. It holds a pool of two tokens, here ETH and USDC, and sets the price with a formula: the two amounts multiplied together must stay the same (x × y = k).
Buy ETH and the pool's ETH shrinks while its USDC grows, so each next ETH costs more. Big trades in small pools get bad prices. The people who supply the tokens earn a 0.3% fee on every trade.
Try it: size a trade and watch where it lands on the curve. Then change the pool size.
Why it matters
Pool prices come from math plus arbitrageurs, traders who buy where it's cheap and sell where it's expensive until prices line up again.
Trade
Pool size
2 · Lending and liquidation
Borrow without a bank, lose your collateral without a warning
Lending apps like Aave let you deposit crypto and borrow against it. There's no credit check, so every loan must be backed by more collateral than you borrow.
Your health factor compares your collateral to your loan. If it drops below 1, usually because the collateral's price fell, anyone can repay part of your loan and take some of your collateral at a discount. That's a liquidation, and nobody asks you first.
Try it: borrow against 10 ETH, then drag the ETH price down or simulate a crash.
Why it matters
Liquidations happen in seconds and in bulk. When prices fall fast, the forced selling pushes them down further.
Simplified rules: you can borrow up to 75% of your collateral's value. Liquidation starts when the loan reaches 80% of it. A liquidator repays half your loan and takes that much collateral plus a 5% bonus.
3 · Risks
What goes wrong in DeFi
Hacks
DeFi contracts hold billions, which makes them targets. Bridges that move tokens between chains have been hit hardest: the Ronin bridge lost about $600 million in 2022.
Depegs
A stablecoin is only as good as what backs it. TerraUSD, held up by an algorithm instead of reserves, collapsed in May 2022 and wiped out tens of billions of dollars.
Oracle tricks
If a protocol trusts a price feed that can be pushed around, attackers can borrow against fake prices and walk away with real money.
Rug pulls
Anyone can launch a token and a pool. Some creators pull out all the liquidity and disappear with it.
Yields that look too good
High advertised returns are often paid in a project's own token. If that token's price falls, the yield goes with it.
Liquidation cascades
One wave of liquidations dumps collateral on the market, which drops the price, which triggers the next wave.
Module 6
Final check
Eleven questions across all five modules. For anything you miss, you'll get a link to the step to revisit.