This box initially appears to be a simple cardboard carton. It can be manipulated and will expand or shrink from matchbox- to carton-size. Moreover, no matter how much mass one places in it, it remains weightless. One must be alone to accomplish this, however; if someone watches the user place an item into the box, that item will weigh its normal amount.
It is possible to expand the box to hold, say, a bowling ball and then shrink it so that it can hold a diamond...and both will still be in there, side by side. The box can be folded in such a way that the contents will be concealed; the number of ways in which it can be folded, realigned, and closed is nearly infinite. People looking in the box will always find it empty; only if they open the bottom flap, refold it, and then open it from the other side (or whatever process reverses what the folder did) will they discover its true contents.