A reconfigurable device comprises flexible bladder that encloses a jammable material. The geometry of the device can be altered by unjamming the jammable material (making it flexible), changing the shape of the device while it is flexible, and then jamming the jammable material (making it rigid). In some applications of this invention, a joint connects rigid arms. The ends of the rigid arms are enclosed in the bladder. By varying the stiffness of the jammable material in the bladder, the stiffness of the joint can be controlled.