Ackerman Paragraph

Here are the topic sentences from "Why Leaves Turn Color in the Fall" by Diane Ackerman. In order to fully appreciate the way the sentences function in the paragraphs, they should be read in context of the essay. Nevertheless, some of the salient features of topic sentences can be identified even out of context.

The stealth of autumn catches one unaware.
This is the first sentence of the introduction. It hints at the wonder and pleasure we experience in the display of autumn leaves and develops the idea that they are a precursor of winter. "Autumn" is the key word identifying the topic. "Stealth" and "unaware" suggest the qualities of the season that the writer will focus on.
Where do the colors come from?
This is a rhetorical question, that is a question used to advance the discussion. There is an implicit understanding that when a writer poses such a question, it is their intention to answer it. Thus, the reader knows exactly where the writer is going. This question implies a reasoned, factual analysis of the process that occurs, and this, in fact, is what the writer gives in terms suitable for a layman.
But as we read on, we can see that this particular sentence is not properly the topic sentence of the second paragraph at all; rather, it is the sentence that launches the discussion of the topic and therefore it is really the organizing sentence for the essay. Ordinarily the organizing sentence is included as the last sentence of the introduction.
The true topic sentence of the paragraph is the second sentence:
Sunlight rules most living things with its golden edits.
What this paragraph in fact does is to tell how the changes in the intensity of sunlight in the solar year cause the leaves to change color and fall. The key word is "sunlight" and the words "rules" and "edits" metaphorically suggest the causative influence of the sun.
A turning leaf stays partly green at first, then reveals splotches of yellow and red as the chorophyll gradually breaks down.
This paragraph continues the description of the process that leads to a change in the color of leaves. This is clear from the content and the time references: "first," "then," and "gradually."
If this paragraph simply continues the description of the process, then why it is necessary to have a second paragraph. Why not put it all together? There are two reasons. First, paragraphing serves as a psychological aid to readers. A solid page of type makes a reader tired before he gets started. "Do I really have to read all that?" he asks. Second, a new paragraph signals a change of topic or a change of focus within a topic.
The paragraph that begins with the rhetorical question explains the process of color change as a function of seasonal change; it is a tree's response to the gradual loss of sunlight. The paragraph that we are currently concerned with narrows the focus by explaining what takes place within a particular leaf. "A turning leaf" is the subject of the sentence and the subject of the paragraph.
The most spectacular range of fall foliage occurs in the northeastern United States and in eastern China, where the leaves are robustly colored, thanks in part to a rich climate.
Not all leaves turn the same colors.
An odd feature of the colors is that they don't seem to have any special purpose.
We call the season "fall,' from the Old English feallan, to fall, which leads back through time to the Indo-Eurpoean phol, which also means to fall.
Children love to play in the piles of leaves, hurling them into the air like confetti, leaping into soft unruly mattresses of them.
But how do the colored leaves fall?
At last the leaves leave.