Abstract: An apparatus comprising: a plurality of sub-units; and one or more connecting members for connecting the plurality of sub-units together in a chain, wherein the plurality of sub-units is held together in the chain by the one or more connecting members disposed between the sub-units, wherein each sub-unit comprises at least one groove through which the connecting member, or one of the connecting members, is movable to allow adjacent sub-units to be rotated around at least one rotational axis relative to each other by a user, and wherein the apparatus further comprises a locking mechanism to lock adjacent sub-units together resisting rotation and resisting disengagement.