Let two bodies pull on each other and the path is always a conic section. We derive the inverse square law from the three laws of Kepler, write an orbit down as six numbers, solve for where a body is at a given moment, and build the cheapest routes from one orbit to another. With three bodies the equations stop having a solution, and yet balance points and resonances appear. Moving figures follow all of it, step by step. Read it as an article, or step through the same pages as slides.