How is this polymorphism ...

[Skhasan]

I am newbie and tryin to learn C#.. I understand what is polymorphism but I dont understand how
this is polymorphic behaviour

1. Create a new Windows application
2. Add a new Button control using the Toolbox, and position it in the center of Form1

private void button1_Click(object sender, System.EventArgs e)
{

((Button)sender).Text = "Clicked!"; //HOW IS THIS Polymorphic behaviour

}