How Derived Class object is added to Base Class objects List

Given the following code, I have inherited a class Circle from Shape:

class Shape
{
    void Draw();
}

class Circle : Shape
{
}

void Main(string[] args)
{
    Shape s = new Shape();
    Shape s2 = new Shape();
    Circle c = new Circle();

    List<Shape> ShapeList = new List<Shape>();

    ShapeList.Add(s);
    ShapeList.Add(s2);
    ShapeList.Add(c);
}

How can c be added into the ShapeList?