@iGreenMonk: You can really scare someone when you yell "Peek-a-boo!". Especially when they're trying on clothes in the fitting room.
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@TheDoorTHEDOOR: An app that detects itself running on other people's phones, then both devices play Random Encounter music. What happens next is up to you.
@HiddleDeeDee: "There is a policeman in here and he will ARREST YOU." And other inspirational things I say to my kids when we're in public.