Locking cap for a drinking vessel

A method of assembling a cap, the method comprising:

• disposing a flexible membrane over at least a portion of a housing, with an annular fin extending downwardly from the flexible membrane and configured to deform outwardly as pressure is applied to the annular fin from beneath the annular fin;
• providing a first slider at least partially disposed in a housing and moveable between a first position and a second position;
• and
• providing a second slider at least partially disposed in the housing in cooperation with the first slider and the flexible membrane such that when the first slider is in the first position the second slider is in a locked position in which a distal end of the second slider presses against and extends the flexible membrane outwardly from the housing, and when the first slider is in the second position the second slider is in an unlocked position in which the distal end of the second slider does not extend the flexible membrane outwardly from the housing as much as compared to the locked position.