In my previous article, I created a basic blockchain that was used to store transactions. The basic blockchain can be tampered with easily. Even though there was a validation mechanism, by updating a block and re-calculating hashes of all blocks after it, the tampered blockchain passed validation. The situation could become worse in the real world because re-calculation can be done in a short period of time with a modern computer. We must come up with a solution to stop attackers from tampering a blockchain. The solution actually is pretty simple and straightforward. That is Proof of Work.