Track 3 · Ethereum & the EVM · lesson 8

Reading a block explorer fluently

9 min

A block explorer is the debugger for a blockchain. Every field on the page is something you now know — this lesson names them in one place.

The transaction page

Status — Success or Fail. Remember that a failed transaction is still on-chain and still cost gas.

Block — the height it landed in, with a confirmation count beside it.

From / To — the sender is recovered from the signature, not declared. If To is a contract, the explorer usually shows the function that was called.

Value — ETH sent with the call. Frequently 0, because moving tokens moves no ETH at all.

Transaction fee — gas used × price. Gas limit & usage shows how much of your ceiling was actually consumed.

Nonce — this account's transaction counter.

Input data — the raw calldata. The first four bytes are the function selector; the rest are the encoded arguments.

"Value: 0" on a transaction that clearly moved a thousand USDC confuses everyone once.

Tokens are not ETH. Moving them updates a mapping inside a contract; no native value changes hands. The transfer shows up in the Logs tab as a Transfer event, not in the Value field.

The contract page

Code — if the source was verified, you can read the actual Solidity. If not, you get bytecode and a strong warning sign.

Read Contract — call any view function from the browser, free.

Write Contract — connect a wallet and call state-changing functions directly. Genuinely useful when a frontend is down, and how you interact with contracts that never had one.

Events — the log history. This is where a token's transfers actually live.

Predict

A contract page shows no source code, only bytecode. What does that tell you?

Choose one answer

The habit worth building

Before approving anything a site asks for:

  1. Open the contract on an explorer.
  2. Check it is verified.
  3. Look at when it was deployed and how many people have used it.
  4. Read the function you are about to call.

Two minutes, and it catches most of what track 2 warned you about.

Check

A transaction moved 1,000 USDC but shows Value: 0. Why?

Choose one answer

Worth remembering

  • Failed transactions still appear on-chain and still cost gas.
  • The sender is recovered from the signature, never declared.
  • Value counts native ETH only; token movements appear in the event logs.
  • Input data starts with a four-byte function selector followed by encoded arguments.
  • Verified source means published code that compiles to the deployed bytecode — check for it before approving anything.