Silverlight checkbox two way binding not working as expected

I have simple issue setting a two-way databinding of a checkbox in Silverlight 3.0. It must be a no-brainer but probably I forgot my brain home today... I defined a Model class to represent my .. 'data'. I implemented the INotifyPropertyChanged interface to enable the UI to see when the data changes. ``` public class Model : INotifyPropertyChanged { private bool _value; public bool Value { get { return this._value; } set { if (this.PropertyChanged != null) this.PropertyChanged(this, new PropertyChangedEventArgs("Value")); this._value = value; } } public event PropertyChangedEventHandler PropertyChanged; } ``` Next I put a checkbox and a button on the .. 'form' : ``` <StackPanel Orientation="Horizontal"> <CheckBox x:Name="check" IsChecked="{Binding Value, Mode=TwoWay}" Content="SomeLabel"/> <Button Click="Button_Click" Content="Test" /> </StackPanel> ``` Then I supplied the data in the constructor : ``` public MainPage() { InitializeComponent(); this.DataContext = new Model() { Value = true }; } ``` The issue is that you have to click twice on the checkbox for it to check/uncheck unless I de-implement the INotifyPropertyChanged. If de-implement it however, then the UI doesn't notice if I change the underlying data. If I remove the Mode=TwoWay bit from the IsChecked binding expression then also the UI won't notice the underlying data change even if the Model is implementing the interface. How can I do to : - Have the checkbox bound to the data at startup - Have the checkbox IsChecked change to modify the underlying data - Have the checkbox detect the underlying data change and update itself?