The Ostomachion — also called the Stomachion or the loculus of Archimedes, "Archimedes' box" — is an ancient Greek dissection puzzle attributed to Archimedes of Syracuse, dating to around 250 BC. A square is cut into fourteen flat pieces: eleven triangles, two quadrilaterals, and one pentagon. The challenge is to fit the scattered pieces back together so they tile the square exactly.
It is one of the oldest mathematical puzzles known. What survives of Archimedes' treatise on it was recovered from the Archimedes Palimpsest, a medieval prayer book written over the scraped-off original text. In 2003, Bill Cutler computed that there are exactly 536 distinct ways to assemble the fourteen pieces into a square, excluding rotations and reflections of the whole square — 17,152 arrangements if those are counted. Ancient writers also describe playing with the pieces freely, arranging them into figures such as an elephant, a ship, or a warrior.
Drag each piece into the square. Select a piece to reveal its controls: turn the rotation knob to spin it, or tap the flip button to mirror it. When all fourteen pieces tile the square with no gaps or overlaps, the solution is added to your collection — every geometrically distinct arrangement counts, and there are 536 to find. The game is free, runs in any browser with no sign-up or install, and your collection is saved on your device.
Also from Aakkagam Games: align3, three men's morris on the Roman and Tamil boards.