Fastener

A fastener comprising two parts separable along a separation axis, at least one part comprising a magnet establishing magnetic force along the separation axis;

• and a mechanical coupling having complementary elements formed respectively on the two parts of the fastener, the mechanical coupling being configured such that movement together of the two parts under magnetic force causes automatic relative rotation of the parts about the axis, wherein at least one of the complementary elements of the mechanical coupling is of helical formation about the separation axis.