If you're friends with someone who isn't a fan of the Harry Potter series, wear the I'm with the Muggle shirt to let everyone who sees you know you're with a non-believer of the magical world.
This shirt adds humor to a wardrobe and it shows people just how much of a wizard fan you are. This tank is also extremely ironic as the chances of the wearer being an actual wizard are pretty slim -- plus, if they were they'd be giving away their secret.
The grey American Apparel top has red writing on it and it includes Harry Potter themes on it. For example, the letter 'M' in the word Muggle is made out of two lightning bolts to resemble the scar Harry has on his forehead.