Because most animation tools are complex and time-consuming to learn and use, most animations today are created by experts. To help novices create a wide range of animations quickly, we have developed a general-purpose, informal, 2D animation sketching system called K-Sketch. Field studies investigating the needs of animators and would-be animators helped us collect a library of usage scenarios for our tool. A novel optimization technique enabled us to design an interface that is simultaneously fast, simple, and powerful. The result is a pen-based system that relies on users' intuitive sense of space and time while still supporting a wide range of uses. In a laboratory experiment that compared K-Sketch to a more formal animation tool (PowerPoint), participants worked three times faster, needed half the learning time, and had significantly lower cognitive load with K-Sketch.