How does it know when to curl? Hair grows from the base. The oldest part is at the tip and the newest part of each hair is at the base. All that stuff most of us see on our heads has already done its growing and is just hanging out as long as it can. Sometimes it just grows straight. What puzzles me is how, when hair curls, it all curls the same direction at the same time as all the other hair around it. But remember hair only grows at the base. All the hair we can see has already stopped growing, yet it curls and waves in unison. How does it do that?