1. As I'm concerned, the first sentence of an essay is the most important part of the introduction. You need to make a good first impression if you want to entice your readers to continue reading.
Here's a list of things to consider before writing your first sentence:
1. Write your essay before writing your first sentence.
2. What is the main focus of your essay? Your piece is about gravity but from what perspective (Newton's theories on gravity, why there is not gravity in space, etc.)?
3. What's the tone of your essay (scientific, creative, etc.)?
4. Make it interesting. Hook the reader.
5. Know your reader. Who are you writing for...your teacher, classmates, etc.?
For some examples of good first sentences, here's a link to some essays that may help you develop your own: