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A lovely technique for helping children take the first steps towards creating their own, unique story , is to ask them to help you complete a story before you tell it.
How come there is a comma',' between story and is? Isn't it that the preceding comma between own and unique is replacement of 'and' so doesn't correspond to the latter one?
Commas don't always have anything to do with grammar.
These two commas are filling different roles. They both indicate pauses in speech, of course, but the first one (arguably) has a grammatical role. It is differentiating between coordinate and cumulative adjectives. Coordinate adjectives use the comma, and they each separately modify the noun - the story would be unique, and it would be their own. Cumulative adjectives, which don't have commas and usually obey the so-called "royal order of adjectives" (I have no idea when or why the royal was introduced to that term), well, accumulate rather than applying separately. One interpretation is that the one nearest the noun applies to the noun, forming a noun phrase, and then the next one out applies to that noun phrase, and so on.
The second one is stylistic. It is not required, but it helps readability - just as the equivalent pause would help people hearing it read aloud to understand - by indicating that you are back in the normal flow of the sentence. This isn't like a parenthetical clause, because there's no separate clause at all, and so there's no requirement for the comma. However, like many usual stylistic choices it's strongly recommended because it actually makes the writing more understandable, reducing the cognitive burden on the reader.
The choice to use commas here at all is stylistic and not demanded by any particular rule of grammar, but including the first one requires the second in order for it to make sense.
Imagine the sentence without the comma you are questioning. It would look like two separate clauses:
(1) A lovely technique for helping children take the first steps towards creating their own,
(2) unique story is to ask them to help you complete a story before you tell it.
The intention of the first comma is not to separate the sentence into two clauses, but rather to separate the two attributes of "own" and "unique" as if they were a list.
Including the second comma marks a return to the continuity of the sentence before the list was introduced. You can see that if the words between those two commas were omitted the sentence structure would still make sense:
A lovely technique for helping children take the first steps towards creating their own
unique storyis to ask them to help you complete a story before you tell it.
