The answer to that is email, which is asynchronous. I.e. if someone is busy they are likely not answering their emails. So if they read their email, they are ac

The answer to that is email, which is asynchronous. I.e. if someone is busy they are likely not answering their emails. So if they read their email, they are actively not working on anything, and will probably easily be able to tell you when you can interrupt them with the question. Or even answer it right there in the email response. Yet people don't do it, because they want their answer now.