State the central idea of a paragraph in a topic sentence. The other sentences should elaborate on the topic sentence with relevant information and evidence. Topic sentences are a key element in developing unified and coherent paragraphs.
Although the topic sentence may appear anywhere in the paragraph, it is most often the first sentence. A topic statement may also contain a transitional element from the previous paragraph.
In the first paragraph of the next example, the topic sentence is the first sentence, stating the subject. However, a paragraph can also lead up to the topic sentence, as in the second paragraph. When a topic sentence concludes a paragraph, it receives extra emphasis and can also serve as a summary or conclusion.
Time became a key word in the language of physics during the seventeenth century. Isaac Newton wove the passage of time directly into his equations, as in force = mass × acceleration. Today, it is difficult for any physicist to examine the universe without thinking of time in mich the same way as the illustrious Briton did more than 300 years ago. Most of the laws of physics continue to be written in the style of Newton; they are designed to show how things change from one moment to the next. Each event under study, such as the path of a ball thrown into the air or the thermodynamics of a melting ice cube, is broken down into a series of freeze-frames that, run like a movie, show how nature works.
Newton had placed a clock upon the mantel of the universe. This Newtonian timepiece ticked and ticked, chiming like some cosmic Big Ben, in step with all the celestial inhabitants, no matter what their speed or position. That meant that a clock situated at the edge of the universe or zipping about the cosmos at high velocities would register the same passage of time, identical minutes and identical seconds, as an earthbound clock. More important, the Newtonian clock was never affected by the events going on around it. Time was aloof and absolute, alike for all as galaxies collided, solar systems formed, and moons orbited planets. Time led an independent existence, separate from nature itself.
--Marcia Bartusiak,"When the Universe Began, What Time Was It?" Technology Review