In Henri Picciotto's Geometry Labs book (https://www.mathedpage.org/geometry-labs/), he proposes this problem: which quadrilaterals can be made with two copies of the same triangle. (Section 6.4 https://www.mathedpage.org/geometry-labs/gl/gl-6.pdf)
The green and purple are copies of the blue, and you can click on a copy to flip it.
