Carpenters worldwide are celebrating Sunday's discovery of a fourth tool: the screwdriver. Until Monday, laborers had but three tools at their disposal: the saw, the hammer and the rotary belt sander. Though many had speculated that a fourth tool might exist, such theories had not been proven true until carpenter Don Kniftle used a screwdriver to insert screws into particle board. 'It occurred to me that my screwdriver could be considered a tool,' Kniftle said. 'A tool is defined as any instrument worked by hand to assist in a task. For example, if I were to use this wrench here to turn these nuts, it too could be considered a... Hey, wait a minute!' 