超级简单的绑定示例

我试图做一个简单的绑定，但我有一些问题。我有一个文本块和一个按钮。textblock被绑定到一个名为"word"的属性。当你按下按钮时，单词的值发生了变化，我想自动更新文本块。这是一个经典的例子，请告诉我我做错了什么:

namespace WpfApplication5
{
     public partial class MainWindow : Window, INotifyPropertyChanged
    {
        private string _word;
        public string word
        {
            get { return _word; }
            set
            {
                _word= value;
                RaisePropertyChanged(word);
            }
        }
        private void change_Click(object sender, RoutedEventArgs e)
        {
            word= "I've changed!";
        }

        #region INotifyPropertyChanged Members
        public event PropertyChangedEventHandler PropertyChanged;
        private void RaisePropertyChanged(string propertyName)
        {
            if (this.PropertyChanged != null)
                this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
        #endregion

    }
}

和我的XAML绑定:

<Window x:Class="WpfApplication5.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <TextBlock HorizontalAlignment="Left" Margin="210,152,0,0" TextWrapping="Wrap" Text="{Binding word}" VerticalAlignment="Top"/>
        <Button x:Name="change" Content="Change" HorizontalAlignment="Left" Margin="189,235,0,0" VerticalAlignment="Top" Width="75" Click="change_Click"/>
    </Grid>
</Window>

RaisePropertyChanged("word");

DataContext = this;