a valve seat having an opening located around a central axis of the valve seat and including a first engaging section and a second engaging section provided around the opening;

a flap being mounted on the second engaging section to block the opening; and the flap being deformable by a user's exhaled air to thereby change the opening from a blocked state into a circulating state; and

a valve cover including a fixed member and a movable member movable relative to the fixed member; the fixed member having a fixing end that is extended through the movable member to connect to the first engaging section, such that a pressing portion of the movable member is located at a closed position to contact with the flap; and the fixed member and the movable member having at least one protruded block and at least one guide groove provided between them, such that the movable member is rotatable about the central axis to an open position for separating the pressing portion from the flap; and

wherein when the pressing portion is moved to the closed position, the opening is kept in the blocked state permanently; and when the pressing portion is moved to the open position, the opening is subject to the user's exhaled air and is changeable between the blocked state and the circulating state.