Rocket tries to press the zipper teeth together with his fingers.
They will not lock.
Raven pulls the slider up. Zzzip! The teeth lock at once.
"What do you notice? The slider did the work," Raven says.
Nova shows a glowing view inside the slider.
"Would you like a hint? Look for a pointy shape," she says.
A wedge is a simple machine.
It has a sharp edge that pushes two things apart.
In a zipper, the wedge also pushes the teeth together.
A doorstop and a snow plow are wedges too.
Inside the slider is a pointy wedge shape.
When you pull up, the two rows meet at an angle.
The slider pushes the teeth together so they lock.
When you pull down, the wedge pushes the teeth apart.
| Sentence | True or false? |
|---|---|
| A wedge has a sharp edge. | ? |
| A zipper slider is a kind of wheel. | ? |
| Pulling the slider down opens the zipper. | ? |
| A snow plow is a wedge. | ? |
Rocket could not lock the teeth with his fingers.
The teeth need a push from the side, one at a time.
The wedge gives that push as it slides along.
The shape of the slider is made for this job.
You found the wedge hiding in the slider. Clever!
Tomorrow we look very closely at real zipper teeth.