Lack of Prior Knowledge

[Taylor]

If a kid wants to be a basketball player, he knows he'll need to be able to dribble, pass, shoot. If a kid wants to be a softball player, she knows she'll need to have the skills to hit, catch, throw, run.
Robotics, and particularly coopetitive robotics (the FIRST family, VEX, etc.) don't really have that prior knowledge for new recruits. People either have no idea at all, or preconceived notions based on (what they think are) similar activities. Thinking one is going to do battlebots, star wars, Wall-E, or computer programming is not necessarily wrong, but it is woefully incorrect.
What does your team do to build a foundation of knowledge within your team, new recruits (student or adult), or the general public so that going into build season, they have an idea of what they're expected to accomplish, and how to go about doing so?

tl;dr: How do you prevent the "let's put a buzzsaw and squirt gun on the robot" comments during brainstorming?