If you39re out to cause mischief then a magic map of the building with the live location of everyone inside will come in handy this is exactly how the marauder39s map provided harry potter with all of the information he needed to sneak around hogwarts the outside of the mug reads quoti solemnly swear that i am up to no goodquot the exact incantation required to activate the map add a hot drink in a mischievous way if you like and the black shroud around the mug dissolves away to