Lather, rinse, repeat

Lather, rinse, repeat is a phrase that is a common part of the instructions on shampoo bottles. It is sometimes also used as a humorous way of saying that a certain set of instructions should be repeated until an explicit or implicit goal is reached, or as sardonic commentary on some people's practice of taking descriptions, instructions or expressions literally and without common sense. Often the phrase is shortened to simply "Rinse and repeat".

The found humor inherent in the phrase is that the instructions, if taken literally, tell the user to lather their hair, rinse the shampoo off, then repeat the process—but they do not say when to stop this cycle. In theory, a person could conceivably keep washing and rinsing their hair forever (or until they ran out of shampoo) as they repeated the instructions over and over. On the other hand, a person using common sense would consider that the word repeat does not refer to itself, but only to the two preceding steps, so the instructions actually mean to repeat the process exactly once, for a total of two washings.

Something else to be considered is the (usually) more appropriate ordering of the phrase, "rinse, lather, repeat," as hair is rinsed before lathered with shampoo, as well as after. However, the reordering of these instructions would leave shampoo in the hair after the cycle stops for failure to include the final 'rinse'.

In the book The Plagiarist by Benjamin Cheever, an advertising executive increases the sales of his client's shampoo by introducing the word "REPEAT" to its instructions.