Prepositions are small words that are combined with nouns into phrases. Prepositional phrases explain time, location, and direction to the reader. As a writer, it is important that you use the correct preposition for the context you are writing. Some verbs in English are paired with a particular preposition to create an expression. Each of these expressions has a particular meaning. Even though there are often two or more prepositions paired with the same verb, each expression has a different meaning and they cannot be used interchangeably. Look at, look for, and look up have the same verb but three different prepositions. The phrases have different meanings and you cannot use look up when you need look for in your sentence. Study the material in Chapter Nineteen carefully and use the chapter as a reference tool when you need to.