Every night before going to sleep, Lisa locks the door of her house and attaches the key to a string. Why?
Lisa was a sleepwalker. Some days before, she had left her house in the middle of the night without realizing. To prevent this happening again, she began tying the key with a piece of string to a bell each night. That way, if Lisa tried to leave the house, her husband would wake up and take her back to bed.