[giardini]

I don't believe the layman has an "ordinary, rational concept of metadata".

The NSA's library metaphor was well-chosen: loose enough to possibly explain but complex enough to mislead. It derailed the conversation.

But I see no utility in the light bulb metaphor you present, except possibly to mislead as most metaphors can do. I would never use it in this context.

The point is to eliminate the metaphors. "Just the facts, ma'am." as Dragnet's Sgt. Friday (didn't exactly) always says.

"why would you want anyone like that to speak a single word?"

To find the truth. If not, to reveal those who lie under oath. To eliminate the metaphors and replace them with facts.