Sometimes when I least expect it one of the boys will come out with a question that I just don't know how to answer. I know very well that they will remember their question when I least expect it and will want to find the answer.
I've started keeping a small piece of paper and pencil in every room (especially the bathroom - because that's where the best questions are thought of!) to capture any and all questions. I also make sure to talk about the questions at some point during the week and make it a point to find the answers with them if at all possible.
One thing I've learned is to treat all of their questions as sincere requests for information. Nothing makes a kiddo feel bad like truly wanting to know something and have their question be made fun of or taken lightly when it shouldn't be!