Screws were metal objects used to mechanically affix two or more objects together. The Doctor often used their sonic screwdriver to unscrew objects.
The Second Doctor uses his sonic screwdriver to remove a screw on Lieutentant Lucke's gun. (TV: The War Games)
The Second Doctor once used the sonic screwdriver to dismantle a Euro Sea Gas pipe. (TV: Fury from the Deep) He later demonstrated to the German officer Lucke that they were from the future by using it to remove a screw from Lucke's gun and then put it back again. (TV: The War Games)
The Fourth Doctor used his sonic screwdriver to unscrew a table on Nerva Beacon. (TV: The Ark in Space)
A device made by the Gendar had screws that reappeared in place if they were removed. (PROSE: A Worthy Successor)