Teachers who don’t know an answer should simply say “that’s a good question but I simply don’t know the answer” or “I don’t have an answer for that but I can do some research and get back to you”.
Such a teacher makes a child feel respected and worthwhile and not put off for having a question. Not everyone has an answer or can think of an answer on the spot. Not every child need an instant answer on the spot. But if they work together they will have a happier outcome.