A question comes from ignorance. You don't have knowledge of that thing, so you ask a question, you seek an answer. A question is a good sign. It shows you seek knowledge. So, no question is stupid.
Children ask the most questions. Are they stupid ? No. They are just very inquisitive. Their minds are open, and they are hungry to learn. Their questions are surely not stupid.
When someone asks a question, don't make him feel he is asking a stupid question. Because when you do that, you create a barrier between you and him. It is the barrier of judgement - where you will judge his questions, and statements. You have put a hole in the relationship. Don't judge a question. Welcome every question with grace, and patience. No question is stupid.