1. Topic Sentences = Topic + Controlling Idea
Before writing any essay or even a paragraph, it is important to think, first about the topic and then what you want to say about the topic. Most often, the topic is easy, but the question then turns to what you want to say about the topic which is the controlling idea. Topic sentences should always contain both (1) a topic and (2) a controlling idea.
Read the following topic sentences. Do they all contain a topic and a controlling idea? Specify them. When your introduction contains a clearly stated topic sentence, your reader will know what to expect and, therefore, understand your ideas better.

2. Topic Sentences = Topic + Controlling Idea
People can avoid burglaries.

3. Topic Sentences = Topic + Controlling Idea
People can avoid burglaries by taking certain precautions.
(The precautions for…)

4. Topic Sentences = Topic + Controlling Idea
There are several advantages to growing up in a small town.

5. Topic Sentences = Topic + Controlling Idea
There are several advantages to growing up in a small town.
(The advantages of…)

6. Topic Sentences = Topic + Controlling Idea
Most US universities require a 550 point TOEFL score.

7. Topic Sentences = Topic + Controlling Idea
Most US universities require a 550 point TOEFL score for a number of reasons.
(The reasons for…)

8. Topic Sentences = Topic + Controlling Idea
Air pollution in Mexico City is the worst in the world.

9. Topic Sentences = Topic + Controlling Idea
Air pollution in Mexico City is the worst in the world for a number of reasons.
(The causes of…) or (The effects of…)

10. Topic Sentences = Topic + Controlling Idea
Fixing a flat tire on a bicycle is easy.

11. Topic Sentences = Topic + Controlling Idea
Fixing a flat tire on a bicycle is easy if you follow these steps.
(The steps for…)

12. Topic Sentences = Topic + Controlling Idea
Travel between the Tehran and Ramsar is enjoyable.

13. Topic Sentences = Topic + Controlling Idea
There are several enjoyable ways to travel between Tehran and Ramsar.
(The ways to…) or (The methods of…)

14. Topic Sentences = Topic + Controlling Idea
Animals in danger of becoming extinct come from a wide range of countries.

15. Topic Sentences = Topic + Controlling Idea
Animals in danger of becoming extinct come from a wide range of countries.
(The different countries… [parts, kinds, types])

16. Topic Sentences = Topic + Controlling Idea
Effective leadership requires specific qualities that anyone can develop.

17. Topic Sentences = Topic + Controlling Idea
Effective leadership requires specific qualities that anyone can develop.
(The qualities (or characteristics or traits) of…)

18. Topic Sentences = Topic + Controlling Idea
Industrial waste poured into Lake Michigan.

19. Topic Sentences = Topic + Controlling Idea
Industrial waste poured into Lake Michigan has led to dramatic changes in its ability to support marine life.

20. Topic Sentences = Topic + Controlling Idea
Industrial waste poured into Lake Michigan has led to dramatic changes in its ability to support marine life.
(The effects of…)

21. Topic Sentences = Topic + Controlling Idea
In order to fully explore the wreck of the Titanic, scientists must address several problems.

22. Topic Sentences = Topic + Controlling Idea
In order to fully explore the wreck of the Titanic, scientists must address several problems.
(The difficulties of…) or (The obstacles to…)

23. Topic Sentences = Topic + Controlling Idea
Summary
A carefully thought out topic sentence will serve two important functions:
First, it will provide you, the author, the means to stay focused on your objective. It's a lot easier to write if you know what you are going to write about!
Second, a clearly stated topic sentence will provide readers with the tools they need to clearly understand what you have to say. These are important reasons to carefully consider topic sentences before beginning writing.