This puzzle is based on the “Weaire–Phelan structure,” the most mathematically efficient way for equal-volume bubbles to fit together to make foam.

See here for video: https://youtu.be/f0dXaUWX-HY

The complete puzzle requires 18 pieces. Ideally, I recommend printing out 16 “Weaire Phelan Tetrastix” pieces and 2 “Weaire Phelan Tetrastix Key” pieces. The “key” pieces have some of the face trimmed back to make it easier to insert them as the final two pieces. If you have difficulty assembling this combination, try using 14 “Weaire Phelan Tetrastix” and 4 “Weaire Phelan Tetrastix Key” pieces.

The Weaire–Phelan structure is made up of two pentagonal dodecahedra (12-sided) and six tetrakaidecahedra (14-sided) that form a translation unit with a lattice periodicity which is simple cubic.

The pieces in this puzzle are each made of six tetrakaidecahedra stacked face to face. The holes inside the complete puzzle are in the shape of the pentagonal dodecahedra.

There is a good introduction to the “Weaire–Phelan structure” here: http://www.steelpillow.com/polyhedra/wp/wp.html

Here is some information on “Tetrastix”: https://en.wikipedia.org/wiki/Tetrastix