Rather than "what are your passions?", which could include activities like motorcycle riding, 'what ideas are you passionate about' dives right into the heart of what would sustainably motivate them.

 

Some people are naturally curious about many things; some are about just a few things.  Curiosity is a natural motivator.  If they find the idea or work interesting, it's a good sign.  It might be the work itself they're interested in, or the learning that they'll obtain through it.

 

Expose them to an idea - do they forget about it?  Or do they get infected, and come back to you?