A good analogy for the cell wall is a sturdy fence or wall surrounding a castle, protecting the structure from harm and only allowing entrance by outsiders through specified gates and openings. In this scenario, the wall also keeps inhabitants safe inside, preventing them from venturing out into potential danger.
An analogy is a method of comparing two things in a way that highlights the similarities, typically by using similar features shared between the two. A cell wall within a living organism is similar to a standard wall in the brick-and-mortar world. Cell walls serve multiple purposes such as protecting the cells from mechanical stress, providing tensile strength and plasticity, preventing water loss, and blocking harmful toxins. Cell walls are not found in humans or animals, but exist in other forms of life such as plants, bacteria, algae and fungi.
The composition of cell walls differs according to the cell species, the cell type and the stage of development. In the analogy of the wall around a castle, that wall could also be constructed of various materials, according to how much protection the castle needs, the inhabitants inside, and how capable they are of defending themselves from dangers within and intruders from outside.