Concentric spheres connected with funnel shapes arranged in a tetrahedral pattern. All the individual pieces, four of which are used for each new layer, are similar. I really like the fractal feel of this one, as it could be continued bigger or smaller indefinitely. Here, I have only shown three layers for clarity. I think a shape like this would make a really interesting terrarium/aquarium because every other layer shares a common space. Would be a challenge to clean though.