A puzzle includes a display showing a plurality of pieces, each of the pieces corresponding substantially to a given region of a composite block, and an input device for allowing a user to manipulate the position of at least some of the pieces so as to construct the composite block. Pieces corresponding to adjacent regions of the composite block have complementary boundaries. A visible property of the complementary boundaries varies with time such that, at any instant, the complementary boundaries between pieces corresponding to adjacent regions of the composite block have matching visible properties. Typically, the variable visible property is the shape of the boundary. Alternatively, it may be a graphic element moving along the boundary.