I find it useful to say: "I don't know, let's figure out how to look it up", or occasionally "Whoops I made a mistake", but not too often. If you make mistakes

I find it useful to say: "I don't know, let's figure out how to look it up", or occasionally "Whoops I made a mistake", but not too often. If you make mistakes regularly on curriculum content, on content that students are expected to perform in a test without mistake, students will quickly lose confidence that you can teach them anything. What is a mistake and what is new material often looks the same, to a student: strange processes that they haven't seen before. They will have to overcome your mistakes in order to figure out what they are supposed to be learning.