I found this site while browsing a Flickr discussion today. It’s a camera simulator that lets you look at a high-light, medium-light, and low-light scene and then tweak the aperture, shutter speed, and ISO to see how the photo will change. You can put the camera into aperture priority, shutter priority, and manual modes and tweak all of the controls. (There are also a couple of other modes available– program and automatic– but they aren’t interesting to us anymore, are they?) There’s also a setting for exposure compensation, which we haven’t talked about before but is used for telling the camera to intentionally under- or overexpose a shot by a certain amount.
And when you’re done, grab your camera and try doing the same thing.
Stop Shooting Auto! 
Just checking out!