Developers spend significant time reading and navigating code fragments spread across multiple locations. The file-based nature of contemporary IDEs makes it prohibitively difficult to create and maintain a simultaneous view of such fragments. We propose a novel user interface metaphor for code understanding based on collections of lightweight, editable fragments called bubbles, which form concurrently visible working sets. We present the results of a qualitative usability evaluation, and the results of a quantitative study which indicates Code Bubbles significantly improved code understanding time, while reducing navigation interactions over a widely-used IDE, for two controlled tasks.