Juan, A mistake some teachers make (I made it early on too) is trying to teach material that I am not completely and thoroughly familar with. Boy, it can show and students are quick to pick up on it. Early on, I made a promise to myself if I was unsurte, I would state that, and add "I'll get the right information". Then, I commit to following through. Nothing worng with saying "I goofed up". Students will excuse humility, but not tolerate excuses or trying to mask misinformation. Barry Westling