This paper concerns the critical role of separable user interface design in teaching object-oriented systems. M206 "Computing: An Object-Oriented Approach" is a large-scale university-level introduction to software development designed from scratch for distance learning, using an objects-first approach with Smalltalk. The course is degree-level, counting as one sixth of a degree, and is being offered in the UK, Western Europe and Singapore. To address the needs of industry we have developed a radical syllabus that adheres to the principle of designing complex systems by separating view and model, and have developed a programming and learning environment to support these ideas. In the paper we examine how separable user interface architectures have guided our teaching of object technology and the design of powerful microworlds that are both usable and extendible by neophytes. The course and relevant teaching with software is outlined and the technical design and pedagogic use of the microworlds and GUI builder tool are described.