For teachers

A free puzzle on a real QR code. Students damage the code one area at a time and find out how far it can go before it stops scanning.

What students learn

Bits and bytes: every square is a bit, and each brick is a byte. Coloring in a square is a bit flip, the most basic form of data corruption.
Error correction: a QR code carries spare data, so a reader can rebuild what's missing. The math is Reed-Solomon coding (1960), the same codes that protect CDs, DVDs and the Voyager space probes' pictures. Every puzzle shows exactly where that protection runs out.
Working within a budget: the most moves without breaking the code. Each puzzle's best possible result is computed exactly, so students have a real target.

Print today's puzzle as a worksheet. It fits on one page, and the brick map is printed beside the code.
In pairs or small groups, one student colors in a light area with a black marker. Another scans the code with a phone camera.
If the phone still shows a link to obsqrity.com, the move was safe. Keep going until it stops scanning.
Compare scores with the best possible result printed on the page.
Talk it over: why did some moves cost nothing, and some cost several bricks at once?

One phone per group is enough. There's no app to install, no sign-up and no ads, and the codes only ever link to obsqrity.com.
Pick a size for your class: the daily puzzle is 21 by 21 squares, Hard is 25 by 25, and Nightmare is 29 by 29. On the print page, choose the size you want.
Every past puzzle can be printed too, so you can print a set ahead of class.
It's free to print and copy for classroom and personal use.

I made obsQRity with a pen and a newspaper ad, and I'd love to hear how it works with real students: hello@obsqrity.com. More answers are in the FAQ.