About this 3D model

A holey ball inside a holey ball. The holey ball is mathematically generated (thanks MathMod). Wonderful conversation piece. Just ask how the inner ball got inside. After printing, it still tricks some people who cannot figure out how to get one ball inside another. There are no joins; the inside ball cannot fit through any holes in the outer ball; and the inner ball rolls around inside the outer ball. It's just made that way.