A hook or attention-grabber is the first sentence of an essay, and its purpose is to catch the reader's attention to encourage them to read further. To accomplish its goal successfully, a hook must have the following qualities:
be surprising or interesting enough to pique the reader's curiosity
be clearly connected to the essay's topic
set the right tone for the rest of the essay (e.g. funny, serious, sad, angry, etc.)
The most common successful techniques for starting an essay are with a short story, a quote, a little-known fact, a vivid description, or a question. Those are all great methods for introducing a topic in an interesting way while setting the essay's tone. In contrast, it is recommended you avoid starting an essay with a dictionary definition or an obvious fact because most readers don't find such methods very interesting.
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