Keratin
Keratin is a tough protein that makes up your hair, nails, and skin.
Think of it this way
Coarse rope is made of many twisted fibers bundled tightly together. Keratin works the same way. Your hair shaft is a bundle of long protein strands packed side by side. Each strand locks to its neighbors, making the whole bundle stiff and hard to break.
Explaining keratin by grade level
Your hair is made of something called keratin. It works like a rope made of tiny threads twisted together. That is why you can pull on a strand of hair and it stretches a bit before it breaks. Different shampoos can make those threads stronger or weaker.