Why does casting an ArrayList 's generic to a superclass not work?

Can someone please explain to me why the line marked //this line gives a compile error (why?) in the following code sample does not work?

import java.util.ArrayList;

public class GenericCastCheck {

    class A{
    }

    class B extends A{
    }

    public static void main(String[] args) {

        A aObject = new A();
        B bObject = new B();

        //this line works fine
        aObject = bObject;
        //this line gives a compile (expected)
        bObject = aObject;

        ArrayList<A> aList = new ArrayList<A>();
        ArrayList<B> bList = new ArrayList<B>();

        //this line gives a compile error (why?)
        aList = bList;
        //this line gives a compile error (expected)
        bList = aList;
    }
}

Specifically, when we say that bList is of type ArrayList<B>, does it not mean that each element of it is an instance of B? If so, then what is the problem in casting it to ArrayList<A>, if we can cast individual instances of B to A?

Thanks.