What is the law of diminishing returns? Give a descriptive example

Please provide the best answer for the statement.

The law states that as additional units of a variable resource such as labor are added to a fixed resource such as capital, beyond some point the additional, or marginal, product attributable to each additional unit of the variable resource will decline. An example would be a factory assembly line. The capital (assembly line) is fixed. As more and more workers (variable inputs) are assigned to work on the assembly line, the output produced by each additional worker is likely to decline at some point. The reasons for this decline are that the assembly line equipment may be fully utilized and beyond some point of production additional workers would only cause problems for the existing workers. Productivity, output per work hour, would fall as more workers are added.