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.
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 :-)
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!