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The security factor
As stated before, each page of the book is designed with 10 lines. So, what happens when we get to transaction number #10? We surely need to go to the next page. However, before doing so, the villagers must validate the transactions and seal the page.
To keep the transactions secured, the villagers decide that each page must be removed from the book and put away on a special wall that will display each page once it is completed. This way, the villagers will be sure that the transactions (hence, the pages) displayed on the wall are true, valid, and correct.
In the blockchain world, the pages are the blocks that contain the transactions. We can compare the wall as the entire blockchain, where all of the validated pages are displayed (in other words, where the blocks are chained together). If one transaction is altered in a block, then all of the following transactions in the subsequent blocks become invalid.
Let's stick to the example to understand how it works.