"Now, the key to the patronus charm is to pick a memory - the happiest memory you have - and focuss everything you have on that memory. You may appreciate that this is significantly more difficult to do when faced with a dementor itself, and I would temper your expectations with the knowledge that even adult witches and wizards struggle with this charm. Does everyone understand?" he was surrounded by nods, some still excited, but the majority looking extremely nervous, "Now - the incantation is "expecto patronum’. Repeat it for me?"