A screw is an inclined plane wrapped around a central core into a spiral shape. Screws can do two things. First, they can convert a force that goes around and around into a force that goes up and down. That's the earliest use of screws, to lift water. You . Second, screws can thread into things like wood or a metal bolt so that the two things are interlocked and can't come apart. That's the most common use of screws today.