Wham! Wham! Wham! It is easy to nail two pieces of wood together. You just knock in nails with a hammer. But fastening two pieces of wood together with a screw is not so easy. The screw has to be turned many times to go into the wood.
A screw has a winding edge called a thread. The thread usually goes from the bottom nearly all the way to the top. When you turn the screw, you wind the thread into the wood.
Turning a screw takes more time than hammering in a nail the same size. But the winding thread of the screw is much longer than the straight sides of the nail. There is more of it to grip and hold the wood. So for some jobs, a screw works better than a nail. It holds things together better than a nail would.
A screw really is an inclined plane that curves around and around. Make a paper screw and see for yourself. Cut a triangle shape as shown from a corner of lightweight card. Mark the square corner with an X.
The long cut edge is an inclined plane. Colour this edge with a crayon or felt-tipped pen. Starting at the straight end, with the X next to the rubber, wind the triangle around a pencil. The coloured edge shows you how the inclined plane winds around the screw to form the threads.
Picture Credit : Google