One example of a symmetric cipher is DES, which stands for Data Encryption Standard. To explain it in simple terms, imagine you are sending a private message to a friend on Facebook Messenger. When Messenger uses encryption, it protects your message so that only you and your friend can read it. With DES, both of you would share the same secret key, almost like a shared password. When you type your message, the key scrambles it into something that looks like random symbols so that no one else, even if they intercept it, could understand it.

When your friend receives the message, their device uses the same key to unscramble it and turn it back into readable text. DES does this by mixing the message through several steps, making it extremely hard for anyone to guess what it originally said without the key. Although newer methods are used today, DES is a simple example of how symmetric ciphers work: one key locks the message, and the same key unlocks it.