Apple Watch and Username and Password

Say you have an app connected to a web service — like Twitter or Facebook — and you create a native Apple Watch app.

How is the watch wearer able to enter their credentials for the service?

Update 11:40 am: The answer, as many people have pointed out, is that this is one of the reasons the Watch requires an iPhone. I’m curious about the details. I’ll be patient.

10 Sep 2014