Generally I think QR codes are a lame stopgap technology. But this application by the Korean retailer Emart is fairly clever. They deployed 3D "Shadow QR codes" that only work during certain sunlight hours, lunchtime specifically. I could imagine this kinda thing being a fun clue in an alternate reality game. "In Seoul, retailer uses 3D QR codes and the sun to deliver discounts only during its quiet times" (Springwise, thanks David Steinberg!)
David Pescovitz is Boing Boing's co-editor/managing partner. He's also a research director at Institute for the Future. On Instagram, he's @pesco.