This article describes the design decisions, technical approach, and evaluation of the animation and interface components for an agent-based system that allows learners to learn by teaching. Students learn by teaching an animated agent using a visual representation. The agent can answer questions about what she has been taught and take quizzes. The agent's performance provides students feedback and motivation to learn, and the overall interaction helps students learn for themselves. The teachable agent uses speech and animation to communicate with the student, and sometimes expresses emotion through her spoken dialog and facial expressions. The technical approach is novel in that it provides a system for creating and customizing an animated agent and associated content in a fast and efficient manner that does not require specialized modeling or animation skills. We evaluate both our interface design and the effects of animated agent on students in a U.S. public school aged 9-11. Results show that the both the new interface and the presence of an animated agent promote positive learning experiences. (Contains 12 figures and 1 table.)