Take a closer look.

Everyone knows the pearl worn by the girl Vermeer painted in Delft. But walk through Delft and you will see dozens more, if you know where to look: a memorial stone with two mistakes, an almshouse courtyard that moved, a plaque on the wrong house.

Pearls of is a city game that rewards you for that looking. The app knows where you are and asks a question whose answer you can only see on the spot. A right answer paints a Delft blue tile, with the story behind it. That way you remember a city not as a list of sights, but as a string of things you found yourself.

Who makes it

Pearls of is published by Van Maastricht Onderwijs & ICT. Marc van Maastricht started out as a biology teacher, has advised schools since 2011 and builds apps himself. You can tell from the app: a game in which you learn something without it feeling like a lesson, and a game leader who keeps a group in hand.

What we hold to

• Sources for every story. Every pearl says where the knowledge comes from: the heritage register, archives, literature.
• Looking on the spot. A spotting question has to work for whoever stands in front of it, not for whoever looks it up.
• Respect for residents. Almshouse courtyards are people's homes: the app asks for quiet and sends no one into a private garden.
• No advertising, no tracking. Your finds and your location stay on your phone.