Advice requested - why doesn't this eventhandler get called?

I'm missing something simple.
I want to hand craft my object hierarchy in code but I'm doing
something wrong. This code displays properly but it does the
MouseDown eventhandler never gets called.

Any ideas?

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
namespace WpfApplication1
{
public partial class Window1 : Window
{
public Window1()
{
InitializeComponent();
Button b = new Button();
b.Content = "PUSH ME";
b.MouseDown += new...

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