"Neither did I, but I learnt it on my travels," Harry replied. "We were just messing about when we were kids in the DA, dabbling unsupervised into some pretty advanced areas of magic, not knowing the depths or potential ramifications of what we were doing. I think it was only through some innate sense of propriety that we managed to avoid touching the Patronus of someone else. It was lucky, too, as we could have done untold damage to each other on a fundamental level."