Here, we use the ngModel directive on the input to define a two-way binding. Because we specified firstName as
the value of the ngModel directive, we connected it to the model property of the same name. A very important aspect
of this process is the fact that it all happens against the current scope. In this particular (and common) scenario,
it would be the scope object used in your controller. When firstName was bound to the text input, it was actually
$scope.firstName that was bound. As discussed earlier in the book, the scope reference is implicit within AngularJS
expressions.