The pillars of any funny commercial are kids and pets and the Doritos Road Chip commercial has both.
Going for the gold in terms of commercial humor, Doritos gets the best of both worlds in this ad. The scene is set in the backseat of a car where a toddler sits eating Doritos chips. Over and over the little dog steals the chip out of her hand and eats it in front of her. Frustrated, she holds the chip up near the window and waits for the dog to lung for it. The dog jumps just as she moves the chip and the dog goes flying out of the window.
This commercial is entertaining because you never really know what kids (or pets for that matter) are thinking so when they do something really clever it's usually amusing.