# FAQ: Introductory Blockchain Concepts - Visualizing Blockchain

This community-built FAQ covers the “Visualizing Blockchain” exercise from the lesson “Introductory Blockchain Concepts”.

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This exercise can be found in the following Codecademy content:

## FAQs on the exercise Visualizing Blockchain

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In the test that follows and I’ve already done, there is an answer in a question which mentions “There is another input that can also produce … (a specific hash)”.
How can it be true? We’ve learned, based on the deterministic property of blockchain, that “the same input will always produce the same output…”.
Moreover, following the hash definiton " A fixed-length string of a varying combination of letters and numbers produced from a specific input of arbitrary size".
So, from the previous two definitions, I conclude that each distinct input (which differs from any other), will produce a different hash.

The hash function produces outputs of a fixed size, lets say 10 characters, but takes inputs of arbitrary size.

Now lets imagine that we calculate the hash of all the possible inputs of 11 characters, since our hash function only produces hashes of size 10 characters, there will be 10 times more different inputs than the possible outcomes. This will lead to different inputs having the same output.

This seem like a problem but in practice the number of different outputs is so large that the possibility of different inputs having the same output (collision) is irrelevant.

I just finished the quiz and I have some questions,

1. so if miner’s purpose is to verify a unconfirmed transaction, what’s in it for them to spend their computing power and electricity to verify other’s transaction?
2. There’s a question that asks “How can a block become valid after being tampered with?”, and the answer was “The tampered block’s hash value changes while the next block still holds the tampered block’s previous hash value.” How does this make sense? I thought the whole point of hash is that it changes whenever any little detail is changed. So if the current block is tampered, thereby resulting in a different hash value, wouldn’t the next block have a different “previous hash” value than the invalid current one?
3. If theoretically hash collisions are allowed to happen with different combinations of transaction data, previous has, and previous timestamp, would it be theoretically possible to produce a fake block with the same hash value and substitute that?
4. If the network takes the longest chain and the 51% consensus as the basis of validity, what’s stopping DDoS style fraud on the blockchain where certain hackers put as many computers around the world as they can as a client with fraudulent blockchain, taking up 51% majority?

sorry for the overwhelming questions, I’m just excited and confused at the same time