Final answer:
Sebum, produced by sebaceous glands, is the substance that prevents hair and skin from drying out by waterproofing and lubricating them, and has antibacterial properties.
Step-by-step explanation:
The substance that helps prevent the hair and skin from becoming too dry is sebum. Sebum is an oily substance produced by sebaceous glands, which are often associated with hair follicles. This oil waterproofs and lubricates the hair and skin, which prevents them from becoming dry. Also, sebum has antibacterial properties that help protect the skin from microbial growth.
Additionally, sebum production increases during puberty due to hormonal changes, which explains why sebaceous glands are relatively inactive during childhood but become more active in adolescence.