On April 22, a billion people around the world are expected to take part in Earth Day 2012 celebrations. Among the anticipated "billion acts of green" will be scores of events for students and schools, from gardening lessons to eco-fairs to solar cooking demonstrations.

All great teachers do great work. And not only that, but they also do different work. Great teachers are always looking to improve practice, steal ideas and try new things -- all in order to meet the needs of their students. PBL teachers are no exception.