Compound words are formed when two or more words are combined to create a new word with a specific meaning. For example, the words tooth and brush come together to form toothbrush, which refers to a tool used for cleaning teeth. Compound words can be written as one word (closed), hyphenated, or as separate words (open), such as ice cream. These words can be categorized into different types. Closed compounds, like notebook, are written as a single word, while hyphenated compounds, such as mother-in-law, use hyphens. Open compounds, like swimming pool, consist of separate words that together convey a single idea.