Seriously, though? Everybody goes to the email analogy. The email analogy really doesn't work.
Not only does it raise more questions than it answers, but it is also not a way people conceptualize social media and it generates the false assumption that the posts themselves exist as the component units of the entire thing as opposed to being tied to the format of the instance.
The thing is you don't even need to bring up interoperability for somebody curious about a specific federated app. In practice, most of the experience doesn't require wrapping your head around that part and somebody can explain the details the first time you get a weirdly formatted posts in your streams.