What should you keep in mind when using a dictionary or thesaurus?

Dictionaries give you exact definitions, multiple tenses, and examples of a how a certain word can be used. Thesauruses give you other ways to say things; synonyms. When using a thesaurus, you should keep in mind that switching out words can often give a negative or positive connotation to a sentence. So if I were to use the phrase "big sister" and the thesaurus said that a synonym for "big" is "enormous," I wouldn't be able to use "enormous sister" as a phrase, since the meaning changed.