Knockout.js how do i bind to a sub property

I know how to bind to a property, but how do i bind to a property like: Parent.Child Using the hello world example on Knockout JS.com: Html: ``` <p>First name: <input data-bind="value: firstName" /></p> <p>Last name: <input data-bind="value: lastName" /></p> <h2>Hello, <span data-bind="text: fullName"> </span>!</h2> <h2>ChildProperty: <span data-bind="text: parentProperty.childProperty"> </span>!</h2> ``` Javascript: ``` var ViewModel = function(first, last) { this.firstName = ko.observable(first); this.lastName = ko.observable(last); this.parentProperty = ko.observable( { childProperty: "I am a child Property" }); this.fullName = ko.computed(function() { // Knockout tracks dependencies automatically. It knows that fullName depends on firstName and lastName, because these get called when evaluating fullName. return this.firstName() + " " + this.lastName(); }, this); }; ko.applyBindings(new ViewModel("Planet", "Earth")); ``` I would like to create a binding to the childProperty. I created a jsfiddle here Thanks