Plank is a six-foot length of solid wood, which can be folded into a coffee table, side table or stool and folded out again to make a plank. It works because its four joints, engineered to high tolerances, cross the board at slight angles, causing it to spiral as it folds.
Heatherwick studio - Plank
(My apologies to my original source. I lost track of you while bouncing around web sites last night...)