Our favicon

I have added a favicon to the blog, which is that tiny graphical element that appears to the left of the blog's name if you have it on a bookmark list, or on your toolbar.  It's rather small, so if you've had trouble making it out, here it is:

Can anyone explain why this favicon is appropriate for this blog?  Or, why I might think so, even if you disagree? You probably need to be an old-timer to get it.