I tried registering with this site on three occasions, but was not successful. I contacted them twice to say there seemed to be a problem. Never heard back, so I gave up.
It's very easy to understand and builds very gently as to not overwhelm you. It seems to be targeted at middle school aged kids, but the information is relevant no matter your age.
The examples the author gives are easy to understand. So far, I'm digging it.
