Sort of starts by being intrigued. You start wondering how a simple emulator works. Then you realise that the simplest high level CPU emulator is just a big switch statement, a block of memory, variables for registers and the Program Counter... Then you realise you need memory read/write functions. Then you start looking at the other systems involved...

It's never ending, it's great fun, and a huge sense of achievement. I'll never forget the first time I saw a BBC Basic prompt when I devved my first emulator.