A system for selectably fastening a female member with a male member adapted to fit inside the female member, the female member having one or more inward-facing recesses and the male member having one or more outward-facing recess, the recesses being of similar cross-sectional dimensions, includes one or more moveable, ferromagnetic locking elements housed within the inward-facing recesses of the female member; and a locking magnet mounted onto the male member, in magnetic proximity to the one or more outward-facing recesses, wherein, in the presence of a magnetic force applied by the locking magnet, mating of the male and female members bringing the outward-facing and inwardly-facing recesses into mutual registration causes the locking elements to move inwardly into the outward-facing recesses so as to extend across both of the inward- and outward-facing recesses, thereby to lock the male and female members together.