Abstract: A nipple includes a nipple body, a peripheral flange, an air intake tube, and an inlet hole. The flange extends outwardly from a periphery of the nipple body, and has upper and lower flange surfaces. The lower flange surface has a recess which is confined by a side wall and a top wall. The air intake tube projects into the recess from the top wall, and has an end part formed with a split for passage of air. The split divides the end part into two openable valve members. Reinforcement ribs respectively connect the valve members to the side wall, thus strengthening the valve members.