The original spec, &quot;Anything that fits into a refrigerator,&quot; is a pretty darn good one. It conforms to Postel&#x27;s law: &quot;Be conservative in what you do, be liberal in what you accept from others&quot;.<p>We can develop a spec in a vacuum, but we can&#x27;t be sure what users actually want from our refrigerator.  Maybe a live penguin actually <i>does</i> belong in there, as in the famous children&#x27;s novel.