Natural granite rock emits a small amount of radioactive particles, the moment of decay of each of them gets detected by a geiger-counter and is true random according to quantum-mechanics.
When the user hammers at the rock and their pattern aligns with the random decay pattern, a solenoid underneath gets triggered and the rock "hammers" back - each time a particle decays.
In effect, the user can establish a form of communication with the true random particle decay of the granite rock. If the user is random enough, both can talk.
tools/sources: arduino, solenoid, geiger counter, granite rock, brass hammer;