What habit does Dill have when telling stories to Jem and Scout?

Dill had a habit of pulling the hair on his forehead. A cowlick is a piece or section of hair that doesn't lay down like the rest. It sticks up or lays the other way.

As he told us the old tale his blue eyes would lighten and darken; his laugh was sudden and happy; he habitually pulled at a cowlick in the center of his forehead.