One weekend my friend Sally, a nurse, was looking after her six-year-old nephew when he fell off a playground slide and hit his head. Worried that he might have a concussion, she checked him all night.

Every hour, she'd gently shake him and ask, "What's your name?" Soon, he began moaning in protest each time she entered the room.

When Sally went in at 5:00 A.M., she found something white on his forehead. Leaning close, she saw a crayon-scrawled message taped to his forehead. It read: "My name is Daniel."