A shim is a tool that is used to bypass padlocks. It works by retracting the spring-loaded catch that restrains the shackle.Like other types of lockpicks, shims can be professionally made or improvised out of easily available materials like aluminum cans.[1]
Higher security padlocks typically use techniques that make shimming impossible. The most common being the use of ball bearings instead of a spring-loaded catch, noticeable because the indentations on the shackle are round instead of wedge-shaped.