It's dark outside. You're alone in the house. You know that, because you have a light on inside, you are visible to any onlookers outside. But outside, nothing is visible to you. Unless it's right against the window. Like a white, pale, hideous, face, pressed against the window, gazing in, gazing in at you. SHUT THE CURTAINS! SHUT THE CURTAINS!

A fundamental and probably primordial fear that affects many, many people (though most severely during childhood). That of seeing a face pressed up against the glass late at night when one is inside.