What is a blockchain?
A blockchain is a type of database that stores information in blocks. Each block contains a list of transactions or data. When a block is full, a new block is created and linked to the previous one, forming a chain. This chain grows longer over time as new blocks are added. The information in each block is protected by special mathematical codes called cryptography.
How does it work?
When someone wants to add new information to a blockchain, the network of computers connected to it must verify and approve it first. Once approved, the information is grouped into a new block and added to the chain. Each block contains a unique fingerprint called a hash that refers back to the previous block. This creates an unbreakable chain because changing one block would break all the connections that follow it.
Why is it secure?
Blockchain is secure because copies of the entire chain are stored on many different computers at the same time. If someone tries to change information in one copy, the other copies will not match, and everyone will know something is wrong. It's like having thousands of identical record books spread across the world. To successfully change the blockchain, someone would need to hack and change the majority of these copies at the exact same time, which is extremely difficult.
Common uses today
Blockchain is most famous for powering cryptocurrencies like Bitcoin and Ethereum, where it records who sent money to whom. However, it's also being used for other purposes like tracking supply chains in stores, verifying property ownership, storing medical records, and confirming the authenticity of digital art called NFTs.
Who controls it?
Unlike traditional databases controlled by a single company or government, many blockchains are decentralized, meaning no single person or organization is in charge. Instead, a network of computers around the world works together to maintain and verify the blockchain. This makes it harder for any one entity to control or manipulate the system. 
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