"Cassie, you cracked?" cried Stacey. "Cassie, where you . . . get back here! Cassie!"

Stacey's words faded into the gray stillness of the January morning as I turned deaf ears to him and hurried after Lillian Jean. "Thanks for waiting up," I said when I caught up with her.

She stared down at me irritably. "What you want?"

—Roll of Thunder, Hear My Cry,
Mildred D. Taylor

What causes Stacey to call Cassie “cracked”?