I get that often myself, though it's about free software rather than open source ;-)  anyway, my usual recommendation is for them to choose a project that they use and find useful and interesting, and that they'd like to improve in some way.  then they'll have a personal interest in putting the time, learning what it takes, and actually exercise the freedoms on software they use.  I think that's a great way to get started.  after you're into it, it's easier to expand into other stuff you'd do e.g. "because it's important", but I guess different people may be motivated by different perceptions