All expository writing prompts give students a reason to write.
However, a prompt that does not contain enough information so students
can respond appropriately is worth than worthless. A poor prompt
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Creates frustration for students.
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Damages your credibility as a writing teacher.
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Makes additional work for you as students keep interrupting
you to get more information about the assignment.
Who needs that grief?
A good prompt
After each National Assessment of Educational Progress (better
known as Nations Report Card), the federal government posts
questions from the test on its website. Ill reproduce one
here with my annotations so you can see that the questions follow
the guidelines for a good writing prompt.
This one is a medium difficulty expository writing prompt for eight
graders.

What is good about this expository writing prompt?
It provides context.
The topic is stated clearly.
The range of acceptable responses is specified.
The prompt says who the audience is.
The directions are clear.
Take a look at the other
sample expository writing prompts from the Nations Report
Card tests. There are prompts for use at eight grade and twelfth
grade. You can use the questions as models or actually use them
in your classes as long as you include the source copyright information.
Youd want to do that even if it were not required because
it makes the assignment look more serious.
A badly-written prompt
A student posting on Yahoo Answers asked for help deciphering a
writing assignment. Im guessing the assignment was for a college
class. I cant imagine any high school teacher mangling a prompt
so badly.
Every person who responded to the students plea for help
had the same reaction: the assignment was not clearly written. Here's
what the prompt said:

Whats wrong with this as an expository writing prompt?
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Theres no context indicated.
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The topic is vague.
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Multiple assertions about the topic are required making
formulation of a thesis extremely difficult.
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The point of the assignment is not indicated.
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No audience is specified.
- The format for the final paper is vaguely described.
Besides all that, the prompt is written as a single 90-word
sentence. Grammar-checking would have pointed out that the sentence
was too long to be understandable.
Give the instructor the benefit of the doubt: the typo might have
been created by the student.
The student who asked for help said shed tried chopping the
assignment apart and had checked the resources the teacher listed,
but still did nott know what to do. Thats the nub of
the problem. The prompt does not give students help getting started.