Or an old teaching technique, teaching by telling a story, in this case introducing encryption.
So, if you're sitting comfortably, I'll begin.
There once was a land in north ruled by a wise kind, Noggin. Despite everything his kingdom seemed to avoid all that rapine and bloody eagle stuff, you
can't trust all the history books, but I digress.
Noggin frequently had to send messages to overseas emissaries, and often they were intercepted by his arch-nemesis, Nogbad the Bad.
In particular, one embarrassing episode of the missus, Nooka*, catching sight of an especially "artistic" woodcut (delivered via the whale road) helped Noggin with his decision.
Anyway, so Noggin searched far and wide in his kingdom for someone skilled in the arts of hiding the meaning of messages. His heralds cried "Noggin seeks a skilled runeatic" (as that was what they were called in those days).
And...one was found, his name was Enogma**, and so skilled was he in hiding the meaning of messages, by his three rotor (powered by ravens) device that Nogbad's evil attempts were thwarted.
And that's the end of the story, next week we'll be listening to Bagpuss's adventures with the Riemann zeta function...
** until his unfortunate death due to a funny tasting apple..prime (no pun intended) suspect was Nogbad, but rumours of over-zealous actions of the part of Thor Nogson's agents do keep cropping up.