I’ve seen some comments over the past couple of days from people wondering what the “killer app” for the Apple Watch will be. I’m pleased to tell you that it’s here—it’s called iPhone.

If you don’t like the implications of calling a hardware device a killer app, then we can say it’s iOS. Either way, some of the great things that will be done on Apple Watch over the next few months will be about the iPhone and the way it interacts with the watch.

Apple Watch is an accessory for the iPhone, that’s very clear. The challenge developers will have is making their app’s UX and UI work and display properly on the watch.

I mentioned yesterday that I really like what Uber has done with its app on the watch. You are initiating something on the watch, not just responding to things happening on the iPhone. This is what we need to see more of from developers moving forward.

One of the mistakes that I made when thinking about the watch was that I’d just be responding to messages—that sounds expensive and boring. Luckily I was wrong.

The idea of watch first, iPhone second is something we’ll have to get used, but it’s also a feature that will make the Apple Watch much more useful for us all.