Negative Curvature

Mel's post about kale got me thinking about negative curvature. Kale is the handiest example of something which is negatively curved: it's not flat because it has too much edge. (Positive curvature has too much middle.) The hyperbolic plane has uniform negative curvature — and you can crochet a model of one! (Directions by David Henderson and Daina Taimina.) I made one out of a lovely celery-green wool which felted so nicely. It's very cool: when you fold it, it bends along a semi-circle.