You can cut a square into four pieces, and reassemble these pieces into a triangle. Or vice versa. This makes a great little puzzle. I’ll show the solution in step 2.

Step 1: Cut the Pieces

From paper, cardboard, plywood, or acrylic cut the square into the four individual pieces using the attached template. I have included a pdf file for printing and a dxf file (in mm units) for laser cutting. The square is about 4" square :-)

Two minute version: Print the pdf file on paper, cut out the pieces, enjoy the puzzle.
Ten minute version: Laser-cut the pieces from plywood or acrylic, clean the edges, enjoy the puzzle.
Twenty minute version: print the pdf template, glue onto a piece of wood/plywood, cut out on a bandsaw or scroll saw, sand the edges, enjoy the puzzle.

Use a material that looks the same on both sides. Flip over some of the cut pieces to make the puzzle more difficult. The pieces are not symmetrical and only assemble with the right sides up!

Step 2: Solution

The puzzle is called Dudeney’s Dissection after Henry Ernest Dudeney. You can look up the history, background, and math on Wikipedia.

What is really cool about it: if you hinge the corners at A, B, and C the triangle can be transformed into a square, and vice versa by folding along the hinges. Another cool fact: if you cut this puzzle from wood, the grain direction will line up for both the triangle and the square solution.

You can build a tray with the triangle/square outline (shown in the last image) to assemble and store the pieces. Make the puzzle for a stocking stuffer or last minute nano-gift! Build a folding trivet or table using the idea with hinges.