Although college essays can offer ideas in many ways, one standard structure for expository essays is to offer the main idea or assertion early in the essay, and then offer categories of support.
Thinking again about how a lawyer makes a case, one way to think about this standard structure is to compare it to a courtroom argument in a television drama. The lawyer asserts, “My client is not guilty.” Then the lawyer provides different reasons for lack of guilt: no physical evidence placing the client at the crime scene, client had no motive for the crime, and more.
In writing terms, the assertion is the thesis sentence, and the different reasons are the topic sentences.
Thesis Sentence (assertion):
The 21st century workforce requires a unique set of skills.
Topic Sentence (reason) #1:
Workers need to learn how to deal with change.
Topic Sentence (reason) #2:
Because of dealing with such a rapidly changing work environment, 21st century workers need to learn how to learn.
Topic Sentence (reason) #3:
Most of all, in order to negotiate rapid change and learning, workers in the 21st century need good communication skills.
As you can see, the supporting ideas in an essay develop out of the main assertion or argument in the thesis sentence.