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.