Like a soap bubble...

Photo courtesy of PublicDomainPictures.net

An excerpt:

The idea hovered and shivered delicately, like a soap bubble, and she dared not even look at it directly in case it burst.  But she was familiar with the way of ideas, and she let it shimmer, looking away, thinking about something else.

~ from Northern Lights (or The Golden Compass)
by Philip Pullman