How decorative keys and official keys work.
The marble in the marble room corresponding to the pits with different depths of key A and key B has a special structure, and a φ 1mm steel ball is installed between the upper and lower marble. When using the A key, the height of the upper and lower marbles plus the height of the small steel ball in the marbles room is just flush with the lock core, so you can use the A key to turn the lock core to unlock; When using the B key, the small steel ball can just be pushed out of the lock core, which means that the lower steel ball is flush with the lock core at this time, so the B key can also twist the lock core to unlock it (this principle is the same as the master-slave key system). But when the lock core rotates to a certain angle, the small steel ball will fall into the cleverly designed blind hole on the lock core (which is the difference from the master-slave key system). Some people call this small blind hole a trap, which is very appropriate. The diameter of this blind hole is larger than the small steel ball, but smaller than the diameter of the marble, otherwise the falling marble will make the lock core stuck and unable to unlock. Once the small steel ball falls into the blind hole, it can only rotate with the lock core and will not run out of the blind hole, which will not affect the use of the B key. However, because there are no small steel balls when using the A key, the marble in the lower layer of this billiard room cannot be flush with the lock core (the height of the small steel balls is different), and it is impossible to twist the lock core to unlock it.