Melissa asked me for her own account on my computer a few days ago. This is significant to a geek such as myself, because I run an almost-exclusively Linux box. I asked her why, and she said she likes some of the programs on my computer, and she wants her own account.
When I asked why the family-access account wasn't good enough, she said that Tatiana has her own account, and she'd like to be able to have her own stuff, just like Tatiana. I interpreted this to mean not just desktop preferences, but also private content. This led to a discussion of computer privacy, and therefore directly to passwords.