How many times has a barely audible "click" awakened my babies? Countless times. They can be jostled from stroller to car seat, back to stroller, back to car seat, into the house, amid noise of all kind, but put them in their bed and close the door... and they KNOW. They know the little buggers, that they are in bed, and that you are leaving them there, and that bed is NOT the place they want to be. Even when they are fast asleep, they KNOW.