Perhaps you read it here: Explainers Shoot High. Aim Low!:

A few years ago, an eminent scientist once told me how he'd written an explanation of his field aimed at a much lower technical level than usual. He had thought it would be useful to academics outside the field, or even reporters. This ended up being one of his most popular papers within his field, cited more often than anything else he'd written.

Addendum: With his gracious permission: The eminent scientist was Ralph Merkle.