Using the design prompt as a guide, brainstorm design activities that illustrate your concept.  Here are some guides for your brainstorming process:

• Write down all ideas, no matter how good or bad.  
• List materials you are interested in exploring, rather than just ideas
• Write analogies to explain your concept
• Use stickies or a “virtual sticky” tool like Prezi or Google Drawings to spatial cluster and regroup ideas
• Switch modes by changing spaces, researching on the web, reading a book, etc.

Some prompts: What are other ways of describing the Core Idea of the concept?  What real world objects or processes are similar to or remind you of your Core Idea? What is the real world applications of your concept? What tasks might people do which depend heavily on your topic? Can you build a model of a real world application out of household materials?

To close up:  Narrow your ideas down to the best three possibilities for design challenges that you want to prototype. At the end of this process you should have your ideas framed as clear design challenges.  A design challenge is a 1-2 sentence description of the goal of the activity, usually in the structure "Build X machine to do Y goal (given Z constraints)."  You may not have a clear idea of the constraints at this point, nor may even need them at all.