You and a friend are building a doghouse. The wooden boards are cut. The holes are drilled. There is just one thing left to do. You have to connect the boards to the frame. You grab a screwdriver and some screws, and go to work.

The first turn of the screw is easy. But then it gets harder to turn. Your dog Ollie tilts his head. He must be wondering what’s going on. You are wondering, too!


Your friend is having an easier time. Your screwdrivers are the same size. The holes are the same size. Even the screws are the same size. So what’s the difference? Then you see it. The ridges, or threads, are farther apart on your screws. Your friend’s screws have a lot more threads. And they are really close together. That must be it!

You try one of your friend’s screws. You have to turn it a lot more times. But, they are easy turns. The screw goes in fast. “That’s more like it!” you say.

It's Simple

A screw is a simple machine. Simple machines help us do work. They help move things faster, farther, or more easily than we can without them.

Simple machines have only a few parts. Some are made of only one part. Nearly every machine is made of one or more simple machines.

A Closer Look

A screw with fewer threads requires more force but takes less time to move.

more threads

fewer threads


When you turn the screwdriver, the screw changes the direction of the force, so it goes forward into the wood.

The Turn of the Screw

Ridges, or threads, wrap around a screw. They form a ramp. That’s what makes a screw do its job.

Try twisting a screw into a piece of wood. Put the tip of a screwdriver into the slit on top of the screw. Turn the handle. That takes some effort. This effort provides the force that drives the screw into the wood.

Each turn directs the force in a circle. The screw changes the directions of the force. The screw moves in a circle. The force drives the screw down into the wood.

A screw with 22 threads means 22 turns and needs less force to move. A screw with 11 threads means only 11 turns but needs more force to screw into the wood.