Running Totals On Same Cell

Place an amount into A2, enter $0 into B2. B2 will continue to change for every change to the value in A2

https://docs.google.com/leaf?id=0BwjqRgkVKt1IN2E4MGNkZTMtNjc4MS00ZmFkLThkOTUtMWIzMWQ2YjJhNDYw&hl=en&authkey=CKmW8pwN