I read this story in a paperback anthology of science fiction stories.
I read the book in or around 1974 but it could have been published
earlier. The story could be much older. The story is about several
groups of people in several prison cells. There is nothing to suggest
that the setting is not Earth. The captors are not seen, food and
drink enter cells through trap doors. There is a soaking wet cloth
strung across the cell. Somehow prisoners determine that their task is
to remove all moisture from this cloth. They believe if they
accomplish this task, they will be freed. This task requires all
prisoners in a cell to work together on it. Only one cell can get
everyone to cooperate on the task. After a Herculean effort they
succeed. At this point the roof of the cell opens. The prisoners
expect to be freed. Instead water is poured in, again by unseen
captors, to soak the cloth again. Story ends on this note.