How does a zipper work?

1. Here is a giant zipper. It has two cloth strips called tapes, each with a row of teeth, and a slider that runs up and down.
2. Look closely at the teeth. Each one has a bump on top and a hollow underneath. The teeth on the two sides are not level: they take turns, left, right, left, right.
3. When you pull the slider up, the teeth run through it. Inside, the slider has a Y-shaped path that bends the two rows toward each other at an angle.
4. As the teeth leave the slider, each bump drops into the hollow of the tooth next to it. Click! Now pulling the sides apart can't open the zipper.
5. To unzip, pull the slider down. A little wedge inside pushes the two rows apart. Bent at an angle, the bumps lift out of the hollows.
6. If one side starts in the wrong place, the teeth meet bump to bump instead of taking turns. They can't lock, and the zipper splits open.
7. Sometimes loose fabric gets pulled into the slider and jams it. Don't yank! Slide down a little, tuck the fabric out of the way, then zip up again.

Tap each part. Watch it work when you find it!

• Teeth. The teeth are made of metal or plastic. Each has a bump on top and a hollow underneath, so it can lock with its neighbors.
• Tape. The tapes are strong strips of fabric. The teeth are fixed to their edges, and the tapes are sewn into the clothes.
• Slider. The slider is a little tunnel shaped like a Y. Moving up, it squeezes the teeth together. Moving down, it pulls them apart.
• Wedge (diamond). Inside the top of the slider is a small wedge. When you unzip, it pushes between the two rows and separates them.
• Pull tab. You hold the pull tab to move the slider. On many zippers it also flips down to lock the slider in place.
• Top stops. The top stops are small clips at the top of each tape. They stop the slider from sliding right off the end.
• Pin and box. On a jacket, the two sides come completely apart. To start, you push the pin on one side all the way into the box on the other. This lines up the teeth.

Drag the slider slowly and watch the teeth join and separate. Then shift one side by half a tooth and zip again. What happens?

Experiment: What happens if one row of teeth is shifted by half a tooth?

1. Zip up with the teeth lined up normally.
2. Unzip, then turn on “Shift one side by half a tooth”.
3. Zip up again and watch the teeth below the slider.

What we found: The teeth have to take turns: left, right, left, right. Shifted by half a tooth, they meet bump to bump, can't lock together and the zipper splits open behind the slider.

Turn on JavaScript to try it yourself.

Push the pin all the way into the box, then zip the jacket to the top without jamming it on the loose lining.

A zipper looks simple, but it's a clever little machine. Each tooth has a bump on top and a hollow underneath. The two rows of teeth don't line up evenly; they take turns, so every tooth on the left sits between two teeth on the right.

The slider is the clever part. Inside, it has a Y-shaped path. When you pull it up, the two rows of teeth slide down the arms of the Y and meet at an angle at the bottom. Bending the teeth at an angle lets each bump slip into the hollow of the tooth next to it.

Once the teeth come out of the slider and straighten up, they're locked. If you pull the two sides apart, the bumps just press harder into the hollows, so the zipper holds. To open it, the slider's wedge bends the rows apart again, and the bumps tip out of the hollows.

That's why alignment matters. If one side starts in the wrong place, the bumps meet bumps and nothing locks. And if loose fabric gets pulled into the slider, it jams. Slide back down a little and tuck the fabric away!

Key words

Teeth (elements)

The small parts on each tape that lock together.

Slider

The part you move to open and close the zipper.

Tape

The fabric strip that holds a row of teeth.

Pin and box

The parts at the bottom of a separating zipper that line the two sides up.

Interlock

When each tooth's bump fits into the hollow of the next tooth, holding the rows together.

Check your thinking

1. What does the slider do when you pull it up?

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   Squeezes the two rows of teeth together so they lock. The Y-shaped path brings the two rows together so every bump drops into a hollow.

2. If fabric jams in the slider, what is the best thing to do?

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   Move the slider back down a little and free the fabric. Pulling harder pinches the fabric tighter; reversing releases it.

3. Why does a zipper split open if the pin isn't all the way in the box?

   Show the answer

   The teeth don't take turns, so they bump instead of locking. The pin lines the two rows up. Without it, bumps meet bumps.

4. Why does a closed zipper hold when you pull its two sides apart sideways?

   Show the answer

   The pull presses each hook deeper into its socket. With the chain straight, sideways force pushes hooks into sockets instead of lifting them out.

5. What opens the teeth when you unzip?

   Show the answer

   A little wedge inside the slider. The wedge pushes between the rows and bends them apart.