I found out another interesting instance on a recent post at Tanya Khovanova's blog. She was teaching one-way functions (functions which are easy to compute but hard to invert - like the product of two large prime numbers), which are useful in cryptography, to eighth-graders.
Her simple "phone-book" example of encoding something like "sun" would be to find last names in a given phonebook that started with the letters "s", "u" and "n", and concatenating the corresponding phone numbers. Putting them together is easy, but inverting is non-trivial.
Here's what happened in her own words:
And one of my 8th graders said, “If I were Bob, I would just call all the phone numbers and ask their last names.”