Learning the tools: shape builder in Illustrator

I have to say that the Shape Builder is the single coolest thing I've learned about in Illustrator.  Drawing curvy lines and making realistic color meshes is neat and all but the Shape Builder makes it quite easy to do exactly what the name says: build shapes.  I built the items below in about a half hour or so:

It's true enough that these shapes are basic (well, the Celtic knot isn't, but I was working on that one from an example to be fair).  However, these illustrate how easy it is to make any kind of shape quickly with this tool.  I have one more object I build with the shape builder during class that is quite a bit more complex.  However, I'm planning to integrate it into another image so you'll have to wait to see that one.