I really like just asking "how do elevators work"? It's so open ended and they can talk about anything from the algorithm, to the mechanical design of elevator

I really like just asking "how do elevators work"? It's so open ended and they can talk about anything from the algorithm, to the mechanical design of elevators, to the UI, to a high-level discussion of all the different components. Really lets you know quickly which parts of systems candidates are interested in.