The interface between a person and a computer can be looked at from either side. Programmers tend to view it from the inside; they consider it their job to defend the machine against errors made by its users. From the outside, the user sees his/her problems as paramount. He/she is often at odds with this complex, inflexible, albeit powerful tool. The needs of both people and machines can be reconciled; users will respond more efficiently and intelligently if they receive meaningful feedback. A “user-friendly” algorithm that covers a wide range of interactive environments and is typical of most operating systems and many application programs is presented.