Why can't I use the methods of an object of class B (a descendant of A) creating an instance like this "A object = new B()"?

Question

Java

class A{
    void printAString(){
        System.out.println("a string");
    }
}

class B extends A{
    void printBString(){
        System.out.println("b string");
    }
}

class Test1{
    public static void main(String[] args){
        A object = new B();
        System.out.println(object.getClass());
        object.printAString();
        object.printBString();
    }
}

An object of class B, but creating it like this: "A object = new B();" i can't use "printBString()" method.
Why is this happening?