When the river rises and floods its banks, it’s useful to have sacks filled with sand to increase the height of the banks of the river. But where do you keep the sacks when the river is not in danger of bursting its banks? After all, you don’t want to be filling sacks afresh every time there is a flood warning. The answer is that you put the sacks in nooks and crannies everywhere, where they sit waiting to be put to use.