Welcome to our humble abode. Today, we shall perform a brief, directed discourse on the workings of the zipper.
Near the turn of the twentieth century, man kind was confronted with a conundrum: how the dickens would they close the flies on their snazzy new Goldrush-era Levis? Already the button fly was becoming associated with a rebel, skater boy type of crowd, and the people were clamoring for something new!
Luckily, someone came up with the zipper. Consisting of two rows of teeth, or "keys," and joined by a y-shaped slider, the zipper is a highly functional fastener. By applying a net force on the slider parallel to the keys, the slider can be moved up or down along the keys, bringing them together or pushing them apart.
The interlocking nature of the keys allows them to resist tensive and shear forces. Since each key is surrounded by other keys for more than 180 degrees, any directional force can be transferred to the next successive key, and so forth. Essentially, no single key is ever subjected to an unbalanced force from stresses placed on the zipper, and as such they can stay in place.