The puzzles belonging to this category are usually solved by opening or dividing them into pieces, including those puzzles with secret opening mechanisms, which must be opened by repeated approximations.
Even those puzzles consisting of several metal parts, connected together in a certain way, belong to this category.
The two puzzles of the image on the right are particularly fun to play during meetings between friends or in family evenings, since they seem to be easily resolvable but, in reality, a lot of people cannot solve them. In this case the problem is represented by the connecting parts that have a conical part that can only be removed with the movement in one way direction. However, each piece has a blocked side so that it could be fitted into the groove of the adjacent part due to its shape. As a result, the other direction is also blocked, so that its groove remains stuck in the side of the next adjacent part.
The so called "secret boxes" or "puzzle boxes with secret opening mechanisms" are extremely popular in Japan and belong to this category. These boxes contain, usually invisible, more or less complex mechanisms that reveal a small gap in the opening. There is a wide variety of opening mechanisms: the barely visible panels that need to be moved, tilt mechanisms, magnetic locks, movable pins that need to be turned to a certain position up and even the time locks where an object must be kept in a precise position until the liquid fills a container.
Some examples are: