Can You Explain The Law Of Diminishing Marginal Utility?

Simply the utility we mean, the power of a good to satisfy human wants, the water has a power to quench one's thirst and a movie has a power to provide enjoyment etc. for our discussion, by utility we mean the satisfaction. As we told above that the classical economists are of the view that the utility or satisfaction depends upon the units of a particular good. It is as U = f (Q) or TU = F (Q).
This is called utility or total utility function. Moreover, as we know that utility can be expressed into numbers, if a consumer drinks a glass of milk, the satisfaction he obtained can be represented into the numbers like 2, 4, 6, 10, 15 etc.
By marginal utility (MU), we mean the net change in total utility by having consumed an additional unit of a commodity.

For example a consumer is using units of apples if the total utility of fist apple is 10 units while the total utility goes to18 units. If he uses the two apples then the net change in total utility or marginal utility is 8.
Now a day, it is said that MU is the derivative of total utility function or it is the slope TU curve.