Cut out a right triangle from paper, then holding a pencil parallel to the hypotenuse wrap the triangle around the pencil. The edge of the triangle as it wraps around the pencil shows how the threads of a screw are constructed. The hypotenuse is an inclined plane when it is seen in two dimensions, but becomes a spiral when wrapped around a cylindrical object - the thread of a screw is also an inclined plane that converts a small rotary force to a large longitudinal force that drives the screw forward.
