Calling functions in extended classes through an object array

Calling functions in extended classes through an object array

I've got an array of objects, some of which use an extended version which
contains a function not available in the base class. How can I call that
function through the array when the array is defined by the base class?
Example
Shape[] shapes = new Shape[10];
shapes[0] = new Circle(10) //10 == radius, only exists in circle class
which extends Shape
shapes[0].getRadius(); //Gives me a compilation error as getRadius()
doesn't exist in the
Shape class, only in the extended Circle class. Is there a way around this?