The Borromean rings form an interesting 3D topological object known as a link. A link is the union of two or more knots, and this is three of the simplest knots (topologically just circles) linked so that they cannot be pulled apart. But if any of the three were left out, the other two would easily pull apart, so it is only in the unity of all three that they are inseparable. I have made several of these in heavy cast bronze (see my university website), so here is a smaller 2.5" model 3D printed. I believe other designers on Shapeways have also made versions of the Borromean rings. There is an interesting variation on this design which I call a Borromean Tripod.