Remember to order two parts (of identical or different colors) to make a complete box.
Challenge your friends to open it! Most of the people tend to use their hands in the same way: they grab objects by opposite faces. In the case of the Tricky Box, those faces will have different colors meaning that they belong to both parts. And if you pull both parts in the same direction, it will not open.
Try it, it is at the same time very simple and not intuitive.
important note: the first time you close the box, it seems impossible to open again because of friction (use water and soap if the two parts seem stuck). So my recommendation for the first time is to close it half-way only and then open and close again several times going each time a little bit deeper. After a while the two parts will adjust perfectly.