Wanna add some advise from my perspective
We start each jam from finding all definitions, synonyms and antonyms of the theme, then we take 15-30 min break, when every member of the team try to find a unique idea or spot some interesting ways to look at this theme or mb even find a game related to one of the topics. After this we start a brainstorm, when everyone writes down and explain every thought, that he came up with. After everyone "came out" with their ideas, we start to discuss if we can actually develop this idea in a few days, isn't it too complicated and so on.
So in this case we have some good ideas but decide that the idea with shadows is pretty cool (in all meanings :D) and not very difficult to implement.
That's more theoretical explanation of how we generally search for ideas for our projects