A while ago I spent some time at home, working on a Rails project. I was very passionate about it and at one point I shouted out: "Finally! The tests are running".
That was the point when my dad got curious about what I was doing and asked me something like "What tests are you talking about?". I tried to showed him my specs. Unfortunately in order to explain the whole specs I also needed to explain ORM, mocking, factories and a couple of other stuff, because it was a Rails project and gotten pretty far by that point of time.
That got me thinking: "If it had been a simple class without any dependencies it would have been a lot easier to explain the tests."
The next logical step for me was: "Just write something simple and explain it next time".
But the most examples out there either include cars or animals. And I didn't want to write classes that are mutable (car that increases speed) or introduce inheritance (penguin inherits from bird).
I created are more abstract but still understandable example: a fraction.