This applet was developed by some wonderfully clever photographers at Stanford. It's an excellent tool to help you wrap your mind around the 3 variables that affect photographic exposure: aperture, shutter speed, and ISO.
Variables that Affect Exposure
During the Spring of 2011, eight high school students in San Francisco were offered an opportunity to learn how to use photography as a tool to create awareness and social change. Over 12 weeks, each young photographer developed a personal photographic story related to the broad issue of health in their communities, and voiced their positions on what makes a community healthy. A selection of this work was presented at the Human Rights Summit, hosted by San Francisco State University on 5/2/11.