Andes is an intelligent homework helper for physics. That is, it replaces the pencil and paper that students would ordinarily use to solve physics homework problems. Students draw diagrams, enter equations and define variables with the same freedom that they have when using paper. Yet, unlike a piece of paper, Andes tells students whether their entry is correct by turning it red or green, and Andes will give principle-based hints when asked. Several semester-long evaluations of Andes showed that students who did their homework on it learned significantly more than students who did the same homework problems on paper.