WPF: How to bind to a nested property?

Question

I can bind to a property, but not a property within another property. Why not? e.g.

Answer 1

All I can think of is that the ParentProperty is being changed after the Binding is created, and it does not support change notification. Every property in the chain must support change notification, whether it be by virtue of being a DependencyProperty, or by implementing INotifyPropertyChanged.