I think you will spend 22 seconds reading this post
I’ve just discovered this really cool web page titled “The Key to Radical Constructivism“. All you ever wanted to know about RC is linked from this page, as it seems.
Have a nice reading session!
Also, a more general page about RC is available here, which is the place where you can also read the marvelous but nonethless free magazine Constructivist Foundations (beware, some articles are hard to follow!)