Edit in JSFiddle

var app = angular.module('myapp', []);
app.run(['$rootScope', function($rootScope) {
    $rootScope.rootMessage = "I am the root";
}]);
app.controller('HomeController', ['$scope', function($scope) {
    $scope.childMessage = 'Hello world';
}]);

<div ng-app="myapp">
    <div ng-controller="HomeController">
        {{childMessage}}, because {{rootMessage}}
    </div>
</div>