Nightmare in the Ghost Dorm

Late at night, a knocking sound echoes in the dorm room, waking me up. Annoyed, I snap back, 'Who is it? Must you knock this late?' I thought someone was at the door. The sound came again. It wasn't from the door, but the balcony window. Our dorm is on the 6th floor. How could someone be knocking on the window from six stories high?