How does a blockchain actually secure transactions?

A blockchain is like a group of friends who all agree on what happened, and no one can change their story without everyone noticing.

Imagine you and your friends are playing a game where each person says what they did during the day. Every time someone speaks up, everyone else checks if it makes sense. If it does, they write it down in a special notebook that only grows bigger, no one can erase or rewrite anything once it's in there.

In this game, each page of the notebook is like a block, and all the pages together are called a chain because they're linked one after another. Each new block has a special code that connects to the previous one, like a puzzle piece that only fits if everything matches perfectly.

When someone tries to change their story later, everyone else checks the code in the notebook. If it doesn’t match, they know something is wrong. That’s how the blockchain keeps things secure, every friend has a copy of the notebook and agrees on what happened. No sneaky changes allowed!