A sack includes a flexible sack body and a mouth that includes two separable lips that form a mouth opening when the lips are separated from one another, and that close the opening when the lips are adjacent to one another. Closing structure is configured to enable a separating force that is applied to the lips to separate the lips from one another to form the mouth opening, and to bring the lips adjacent to one another to close the mouth opening in the absence of the applied separating force.