For more than ten years we've been running a popular MATLAB programming contest built on the strange premise of openly sharing all code with your competitors. We wanted to grow this contest into something that people could play anytime, something that they could grow and modify themselves. In turning our contest into an always-on quiz game (called Cody http://www.mathworks.com/matlabcentral/cody/), we realized we now have a test-driven learning platform in which instruction and assessment happen spontaneously and in the wild.