This best-known puzzle of the "sliding block " category will be new to anyone whose grandparents didn't have the foresight to save the original version. In its pristine state, tiles numbered from 1 to 15 are arrayed in sequence in a 4x4 frame that has one empty space to permit sliding the tiles into new arrangements. The object is to mix up the tiles, then restore them to their original state, or into 40 alternative configurations illustrated, eight of which are supposedly impossible. This is a keeper; save it for future generations