Great ! Thank you everyone. Everything is "clear" now 

I didn't understand this...

You unlocked something in my head.

Still something I don't understand. If I send a bitcoin now. The transaction will be accepted in the next block because the miners are already working on the block at the moment, right ? Or can they add unconfirmed transactions while they are solving the block ?

I think I begin to understand

So, when I send a bitcoin to somebody, the transaction goes to the mempool. The mempool is like a big pool where the transactions wait to be picked up. All the miners have access to the mempool to choose the transactions they want to put in their block.

1) When a miner pick up a transaction in the mempool, the others miners can't take it ?
2) If all the miners can choose what they put in the block, they are all searching for a different hash to solve the block, right ? I'm saying this because with this simulator (https://blockchaindemo.io/) The "Data" input represent the different transactions in it, no ?

Thank you !


They are all working together, right ? So they all choose the number of transactions in the next block, and then they ALL work to find the hash ?

Hello !

I have some questions about the bitcoin blockchain, and I'm pretty sure to find them here  .

1) If I check a blockchain explorer, I see that some blocks are mined very fast and others not. Some blocks have only 100 transactions in it (https://btcscan.org/block/00000000000000000007465f8d675de9de8f075d4da9b9fb843efc7600d9a944) and others have more than 3000. Why ? It's a question of luck ? Or is the network less used at this moment ?

2) It takes approximately 10 minutes per block to be mined. If I make a transaction, my transaction is validated before a new block is mined ? If not, do I have to wait 10 minutes between 2 transactions ?

2.1) Every information added in a bloc that will be mined change the hash. Is some transactions added in a block DURING the time the miners are working to find the right hash ?

I hope you understand