Humans will say "NO" at particular point

Utility is described as the level of satisfaction that a customer or consumer derives by consuming a particular product at a given period of time.

                                                               

Law of diminishing marginal utility states that the more and more you consume a particular product at a particular period of time the less additional satisfaction you derive from it.

Example: You are very thirsty, the first glass of water you drink gives more satisfaction than the second one, and the level of satisfaction decreases as you go on consuming more water, the satisfaction level will be zero at a particular point of time and go negative you don't want to drink water.