I've found that it helps to, rather than going directly into the mathematics of it all, explain to somebody that their password is going to be cracked by a comp

I've found that it helps to, rather than going directly into the mathematics of it all, explain to somebody that their password is going to be cracked by a computer rather than a human, and computers 'think differently'. Thus, you need to make things that seem complex to computers, not things that seem complex to humans. The same applies verbatim for people trying to roll their own 'clever' encryption schemes.