How about it? And while we're at it, let's talk about us—also he, she, him, her, they, and a slew of similar words, the small conveniences that refer to things or people we'd rather not mention by name.
These words are called pronouns because they're substitutes for nouns (pro means "for" or "in place of"). Most of the time we can decipher the shorthand and figure out what it is and who they are. When the words in a sentence are in the right order, there's no doubt about it. Even when the word order is iffy, logic and context usually help us fill in the blanks—but don't count on it.
This sentence leaves no doubt: The La-Z-Boy is moth-eaten, so Homer's replacing it. Here, it can only mean the La-Z-Boy.
Add another noun, though, and the shorthand is blurry: The upholstery on the La-Z-Boy is moth-eaten, so Homer's replacing it. Is it the La-Z-Boy or the upholstery? Will the mystery noun please stand up? You might mean this: The upholstery is moth-eaten, so Homer's replacing the La-Z-Boy. Or this: The La-Z-Boy is moth-eaten, so Homer's replacing the upholstery.
It is one of those creepy-crawly words that sneak up on writers. Every time you write it, imagine a reader asking, "What is it?" If it isn't obvious, either ditch it or rearrange the words.
Be careful with sentences like this, with two or more nouns in front of an it: Philippe kept his opinion of the painting to himself until it became popular. Until what became popular, the painting or his opinion? Make sure the reader knows what it is. Try this: Until the painting became popular, Philippe kept his opinion of it to himself. Or in case he's an art critic: Until his opinion of the painting became popular, Philippe kept it to himself.
If you'll pardon the déjà vu all over again, here's one more example: Yogi's book about the World Series was sold even before it was completed. Before what was completed, the book or the World Series?
One way to fix the sentence is to drop completed and use a more precise verb that clears away the fog: Yogi's book about the World Series was sold even before it was written. Another solution is to add he, making clear who did it: Yogi's book about the World Series was sold even before he completed it.
If one shorthand word can gum up a sentence, imagine what a whole pack can do. Try to identify the pronouns in this pileup: Fred told Barney he'd ask a neighbor to feed his pterodactyls, but he forgot, they died, and now they aren't speaking.
Whose pterodactyls? Who forgot what? Who (or what) died? Who's not speaking to whom? When you use pronouns, you know the cast of characters. Readers won't know and shouldn't have to guess. This might be what the writer means: Fred said he'd ask a neighbor to feed Barney's pterodactyls, but the neighbor forgot, the pets died, and now Fred and Barney aren't speaking. It's not elegant, but at least we know who did what.
Even a short sentence can be confusing if it has a mystery pronoun: Duke said Boomer broke his nose. Since two guys are mentioned, we don't know whose nose was broken—Boomer's or Duke's. If Boomer took the blow, we could write: Duke said Boomer broke his own nose. If Duke's face was rearranged, we might say: Duke said his nose was broken by Boomer. (A passive verb comes to the rescue.)
Those solutions aren't as economical as the original sentence, but clarity comes first. Sometimes we can solve a pronoun problem by using a different verb altogether: Duke accused Boomer of breaking his nose.
Oh, one more thing about fuzzy pronouns. Don't substitute the former and the latter to make your meaning clear (Duke said Boomer broke the latter's nose). The result is annoying and pretentious. A good rule of thumb is to avoid the kind of pompous language used by people you'd like to punch in the nose.