From the website
" Challenge-based learning (CBL) is similar to problem-based learning, but with CBL, students formulate the challenges they will address. Through a process of discussion and research, students identify a selection of questions that might be workable for their project, work on solutions, and publish those solutions online. In this way, CBL provides the satisfaction that comes from figuring out both the issue to be tackled and the solution to it, even though CBL requires a heavier time commitment than more traditional academic activities. Students gain meaningful skills through these projects, including how to share work, collaborate, organize, and express themselves more effectively."
Via Jim Lerman