As you can see in the photograph below, Dave has been keen to avoid being surprised lately. Fortunately for Dave, he hasn't faced any true threats, but that doesn't mean he is complacent. He knows he must keep vigilant, but he can't keep his eyes looking in all directions. Dave's most vulnerable direction is to his rear, something that he resolves by sitting immediately in front of a barrier most of the time. (That is why you rarely see Dave napping in the middle of a room. Never sit with your back to the door, Dave says.) After his rear, his most vulnerable direction is directly above him. But after being surprised too many times from this direction, Dave knows to be wary of threats from above.