Fusion extensions are a method of hair extension that involves attaching the extensions to the natural hair using heat. The extensions are made of human or synthetic hair and are pre-tipped with a small amount of keratin, which is the protein that makes up the hair. A heated tool is used to melt the keratin, which is then attached to a small section of the client's natural hair. The melted keratin bonds the extension hair to the natural hair, creating a permanent attachment.