Watches and clocks are complex machines with many small parts, and repairing them requires precision and delicacy. The ability to locate and correct defects is an important and necessary skill for watch and clock repairers. They employ a standard, systematic procedure to track down defects, sometimes using information from customers about the history and previous repairs of the timepiece. Some problems arise from incorrect replacement or improper fitting of parts. Careless pushing, pulling, or turning the winding device can also cause problems by making parts too tight or too loose, or permitting dust to enter the mechanism.