Dudeney's Dissection
In honor of a new proof by Demaine, Kamata, and Uehara that Henry Dudeney's dissection linking an equilateral triangle and a square is minimal. (Cf. arxiv.org/abs/2412.03865)
You can free play, or set the frames to get the triangle or square outline. The goal is to arrange the pieces one way to make an equilateral triangle and another way to make a square.
Mathworld notes that in his April 20, 1902 column , Dudeney gave a five-piece solution while noting that C. W. McElroy of Manchester had found a four-piece solution. In the following column, Dudeney presented the four-piece solution without a clear indication if it was due to Dudeney or McElroy.
The dissection is sweet as it's not just a minimal dissection, it's hinged as well.