Concepts are tested with CAD models, which are used to make zipper molds on steel plates. The zipper features microscopic J-shaped grooves (“hooks”) and arrowhead-like stems that interlock. Look closely at a double zipper bag and you’ll see this line of tiny teeth on the upper row, toward the lips of the bag. Running your thumb and forefinger along the track clasps the hooks around the stems. The zipper clicks at about 50 decibels to let you know you’re doing it right. But misalignment due to your clumsiness can create gaps.

Research and Development at it's best! Who would have thunk it. (Bold is my emphasis)

via Wired