A zipper is a fastening device made of two strips of fabric with interlocking metal or plastic teeth that join together when you pull a small sliding piece called the zipper pull up the middle. Push the pull down, and the teeth separate, opening whatever the zipper was holding closed.

Zippers appear everywhere: on jackets, backpacks, jeans, sleeping bags, and countless other items. Before zippers became common in the 1920s, people relied mainly on buttons, which took much longer to fasten and unfasten. Try to imagine buttoning twenty tiny buttons every time you wanted to put on a winter coat!

The interlocking teeth create a surprisingly strong seal. Each tooth has a tiny bump on one side and a hollow on the other, so they hook together as the slider passes over them. The slider's wedge shape forces the teeth to mesh together when going up and pulls them apart when going down.

When a zipper gets stuck, it's usually because fabric or thread has caught in the teeth, or because the teeth have bent out of alignment. Sometimes you'll hear someone say they zipped through their homework, meaning they finished it quickly and smoothly, like a zipper sliding easily along its track.