On this week's Please Explain we'll find out how the human eye works and how it enables us to see the world around us. Dr. George A. Cioffi, Chairman and Ophthalmologist-in-Chief of the Department of Ophthalmology at NewYork-Presbyterian Hospital/Columbia University Medical Center, tells us how to keep our eyes healthy; why our vision changes; and how problems like glaucoma, cataracts, and macular degeneration affect our vision.