For someone that is truly a beginner in cryptography it is helpful to understand at a very basic level how encryption/decryption works.

Using something simple as a Caesar cipher or ROT13 you can show how plaintext undergoes a transformation using an algorithm to create the cipher text and how that same algorithm is used to convert the cipher back to plaintext.

This simple idea provides the basis to discuss other cipher techniques (and their shortcomings); as well as provide an early example of symmetric-key algorithms.