The invention relates to a straw comprising a tube (11) extending between a first end (13) and a second end (14) and comprising a stopper (12) placed in the tube near the first end thereof and extending between a first end (17) facing the first end of the tube, and a second end (20) facing the second end of the tube, said stopper and said tube being configured such that, after a liquid substance (22) has come in contact with the stopper via the second end thereof, the stopper blocks the passage of the liquid substance therethrough and is, by pushing on the first end thereof, slidable into the tube toward the second end thereof. Moreover, the stopper comprises, on the side of the second end of the tube, a barrier pad (16) consisting of hydrophobic threads over the entire cross-section of the tube.