A closed zip isn’t held shut by the slider. Each tooth has a bump on top and a hollow underneath, and each one sits nested between two teeth from the other side, so pulling the two halves straight apart only jams them tighter. The teeth can only meet, or part, when the two rows come together at an angle, and that is all the slider is: a small Y-shaped channel that bends them. Going up, it folds the rows in and they lock; going down, a wedge in the fork of the Y pries them apart. Gideon Sundback worked this out around 1913, and ten years later a rubber-boot company gave it the name zipper. So the one part of your coat you actually hold on to holds nothing at all. Its whole job is knowing the angle.