A hypotrochoid is the curve traced by a point fixed to a circle rolling inside a larger circle. The old mathcurve.com page shows them as static GIFs. I wanted to play with the parameters live, so I built a small single-page app: sliders for the two radii and the pen distance, an animated pen that draws the curve and keeps looping, a rainbow mode, and shareable links that encode every setting in the URL. No build, no dependencies, one HTML file. Play → · Code →