Asking an expert a real question is a compliment. Plenty of them rarely get asked anything beyond small talk.

People who know a great deal about something often spend most of their time among people who never ask. A real question, one that shows you have thought about the thing, reads as interest in their work rather than as an imposition, and it is frequently the best part of their week. The worry about wasting their time usually has the situation backwards.

Write a proper question for somebody whose work you admire and send it, or ask it after a talk. Keep it short and specific, and say what you already tried to work out yourself.