A classic, unassuming design masks the baffling difficulty of the Double Zero Marble Puzzle. The natural color palette and symmetric, simple shapes make this wooden puzzle feel like something you might discover in a hip, Scandinavian preschool. Plus, the mechanism is simple enough for a young child to operate. Looks can be deceiving, however. This sequential movement puzzle is comparable in difficulty to a Rubik's cube (with no easy-to-follow YouTube tutorials). The sliding middle panel exchanges four marbles at a time, which means you'll need to think way ahead in order to separate all the colored marbles into their matching sections. To make matters worse, half the marbles are double units that can't be separated.

We'll be totally honest, we haven't solved this one yet. It's a considerable step up in difficulty from the .