"What is an interface?" reminds me of one of my favorite interview questions: "What happens when you open a browser and visit a webpage?". There’s no single right answer and when asked in the right way I find it helps surface knowledge and skills candidates might not otherwise bring up. Some go into OS internals, others talk about networking, some focus on UI and page load performance. Even when candidates expect the question it still reveals how they think and what they spent time reading up on.