Final answer:
When using conditioner, heat is recommended to open up the cuticle of the hair shaft in order to allow the conditioner to penetrate the cortex for better moisturization and nourishment.
Step-by-step explanation:
When using conditioner, it is sometimes recommended to use heat to open up the cuticle of the hair shaft. The hair shaft has three zones: the outermost zone called the cuticle, the middle zone called the cortex, and the innermost zone called the medulla. The cuticle is the protective outer layer of keratinized cells that covers the cortex. By using heat, the cuticle can be temporarily lifted, allowing the conditioner to penetrate the cortex and provide better moisturization and nourishment to the hair.