Betty's comment is exactly the problem. It's that kind of surprised reaction that makes people feel dumb. Just answer the question without the judgment.
Honestly, learning something new is a process for everyone. Maybe they just never had a reason to figure it out until now. It's not like we're born knowing how to do everything. There's a first time for all of us, and asking is how we learn. Shaming someone for that just makes people scared to ask questions later. We should be helping each other out, not making people feel bad for not knowing.