The reader needs to know this and it is your job as the writer to paint the appropriate picture for them.
To do this, it is a good idea to provide the reader with five or six relevant facts about the life (in general) or event (in particular) you believe most clearly illustrates your point. The importance of this step cannot be understated (although it clearly can be underlined); this is, after all, the whole reason you are providing the example in the first place.
Despite the fact that, as Shakespeare said, "the pen is mightier than the sword," the pen itself is not enough to make an effective writer.
In fact, though we may all like to think of ourselves as the next Shakespeare, inspiration alone is not the key to effective essay writing.
Active voice, wherein the subjects direct actions rather than let the actions "happen to" them – "he scored a 97%" instead of "he was given a 97%" – is a much more powerful and attention-grabbing way to write.
At the same time, unless it is a personal narrative, avoid personal pronouns like I, My, or Me.
Only then, with the reader’s attention "hooked," should you move on to the thesis.
The thesis should be a clear, one-sentence explanation of your position that leaves no doubt in the reader’s mind about which side you are on from the beginning of your essay.
Finally, designing the last sentence in this way has the added benefit of seamlessly moving the reader to the first paragraph of the body of the paper.
In this way we can see that the basic introduction does not need to be much more than three or four sentences in length.