In a piece of writing, a hook sentence is designed to grab a reader's interest and make him want to continue reading. While engaging the reader, the hook also introduces the main topic of the work. Typically, it is the first sentence of the first paragraph.
Writers often use quotes, statistics, unusual facts, questions, anecdotes or interesting details for their hooks. Although it is the first sentence, many writers do not write the hook until much of the essay has been finished. By this time, the theme is well established and a writer has a good grip on the topics; therefore, finding a compelling phrase or idea that draws readers into the work is easier.