The straw comprises a tube ( 11) and a cap (12) perméable to gases and liquid-tight, said cap (12) being arranged in the tube ( 11) in the vicinity of one end. The cap (12) comprises a magnetic component configured such that a magnet (21), placed flush with the tube ( 11) at a right angle to the cap (12), and then moved on a horizontal surface (20), is then fbllowed by the straw (10).