A flexible container has a container body with container walls that define a container interior. A container socket is connected to the container body and has a container opening through which the container interior is filled. Carrying loops are connected to the container body for lifting the container. The container socket has an initial retracted position in the container interior and is transferable from the initial retracted position into an upright filling position. The container socket is at least partially comprised of an elastically expandable material. The flexible container is filled by inserting into the container socket a filling tube, contacting with the filling tube the container socket in the initial retracted position, and transferring the container socket to the upright filling position and keeping the container socket secured on the filling tube in the upright filling position. The bulk material is introduced through the filling tube into the flexible container.