Explain It Like I'm Eight is a new video series from Advertising Age that looks to bring a little clarity to our jargon-filled world. Today's topic is viewability. Defining what constitutes a viewable ad is much more complicated than you'd think, with standards dictated by 14 pages of guidelines from the Media Rating Council and IAB. Still, marketers pay for the ads that can't be seen. But why? Below, an explanation so simple a child can understand.