Human: I have a question about the wave-particle duality of quantum mechanics. Why is it that a particle can be in two places at once? Assistant: 
The wave-particle duality of quantum mechanics is a fundamental principle of the theory. It is a consequence of the fact that a quantum system can be in two states at once. In other words, a particle can be in two places at once. This is a consequence of the fact that a particle is a wave.