In elementary school or high school, we often use stuff that has never actually been proven (in that class). For example - Pythagoras' theorem. - Addition of natural numbers is associative. - Every number can be uniquely (up to order) decomposed in prime factors. Accepting such a things really annoyed me, I would always ask why something is true. The answer that most teachers gave me was "can you find an example where it doesn't work," sigh. I had to wait until university to actually see a proof for such a things... So, were you ever annoyed that something wasn't proven in school?? And what would have liked to see a proof/reason of??