I obtained Growing Triangle from Andreas during the Puzzle Exchange at IPP35 in Canada two years ago. In Andrea's own words....
"This puzzle is inspired by "London Squares" which was designed by Li Zhunyou and exchanged by James Kerley at IPP34. Although I very much enjoyed the puzzle I spotted some "usability problems" and I want to fix with this design"
Growing Triangle is precision laser cut from 6mm clear acrylic< Made by Mr Puzzle, it consist of 12 irregular shaped pieces. Each of the pieces are also etched with markings to indicate the number of triangular units within. The finishing touches include a nice red drawstring pouch.
There are 11 challenges to the puzzle, simplest being to take 2 of the 12 pieces and form an isosceles triangle of 4 units length per side. Then take 3 pieces and form a triangle of 5 units per side, 4 pieces to form triangle of 6 units per side until all 12 pieces are used to form the largest triangle of 14 units length per side. Each challenge offers a unique solution and as you would imagine, moving from 2 to 12 pieces becomes progressively harder and painful. I have shown just the first two solutions here as an illustration of what this puzzle is about.
So far I have grown my triangle to 11 units length per side using 9 of the 12 pieces. But I have since remained stuck at this level (but I am still trying).
Oh, and did I mention that you can use Burr Tools to solve all 11 challenges of the puzzle? :-)