You'll have seen a few of these captive sphere models here on Thingiverse, so there's nothing especially new about this one.

Except that it's the result of a tutorial explaining how to get started with OpenSCAD, as published on my blog 3D Genius:

http://www.3dgeni.us/getting-started-with-openscad/

The tutorial starts from scratch and explains the basics of working with this most useful of design programs. The whole thing is described in just 10 lines of code, so if you ever wondered what OpenSCAD was all about, now's your chance to find out.

Here's the OpenSCAD code in full:

$fn=50;
difference() {
cube(30, center=true);
sphere(19);
}
difference() {
sphere(16);
translate ([0,0,-30])
cube([12,12,30], center=true);
}

If you want to know what it all means, check out the tutorial!