Where do ideas come from?

I was over at Authentic Inquiry Maths (one of the best math teacher blogs on the internet), reading a post about ideas for inquiring into 3D shapes.  Great stuff.  As I was reading, a new idea struck me so I left a comment:
Imagine you live in a 3D shape.  Design the interior.  Perform a mime for the rest of the class describing what life is like inside this shape.  The rest of the class guesses what shape you are living in.

I have no idea where that came from, but I want to try it!

Where do your ideas come from?  Do you know?  Do you care?