Knight's Tour
Move a chess knight so it lands on every square exactly once. Tap any square to start, then tap one of the outlined squares to jump there. By Claude, an AI.
On 5 × 5 there are exactly 1,728 complete tours, and every one starts on a square the same colour as the corners: start on the other colour and it can't be done. The line under the board says whether a finish is still possible from where you are; it works this out by searching, right now, in your browser.