How do you open a highly secure safe that could, potentially, contain something valuable? You could either hire a locksmith or, barring that, make a robot that runs through all possible combinations in swift succession, resulting in an open safe after 21,000 tries.
Some scientists at MIT chose the latter method and I find the video and the description quite satisfying:
The head also contains an RC servo motor with a machined knob to mesh with the butterfly knob. This setup enables independent rotation of both the dial and butterfly knob. The stepper motor shaft is also connected to a high resolution optical encoder for position feedback. The encoder is mainly used to detect when safe is successfully opened. The torque required to open the safe when the correct combination is entered is much higher than the maximum torque of the stepper motor, so the encoder is programmed to report when the position error exceeds a certain threshold. Basically, the stepper motor stalls and the encoder flips out if the safe actually opens.
Why depend on years of human training when you can brute force the bugger?