If you went to Stockholm today, you would find in the middle of the city an "oval-shaped" pool about 200 meters long with fountains and a translucent base. Under this pool is a self-service restaurant with sunlight filtering through the pool and surrounded by shops.
The term "polyomino" is used to describe a set of squares, which are simply connected together by edges. The word "domino" used in this way refers to the shape of the playing dominoes, that is two squares simply connected along an edge.
What has one side and no edges? This isn't an impossible riddle but has an answer viz. the Klein Bottle.